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FIG. 323. PLANO-CONVEX LENS WITH LEAST ABERRATION, 809 




FIG. 322. PLANO-CONVEX LENS WITH GREATEST ABERRATION, 




FIG. 320. CROSSING OF LIGHT RAYS WITH GREATEST ABERRATION, 




FIG. 32.1A. LIGHT CONK WITH THE RADIANT ABOVE THE OPTIC Axis, 57 




OPTIC PROJECTION 



PRINCIPLES, INSTALLATION AND USE 



OF THE 



MAGIC LANTERN 
PROJECTION MICROSCOPE 
REFLECTING LANTERN 
MOVING PICTURE MACHINE 




FULLY ILLUSTRATED WITH PLATES AND WITH 
OVER 400 TEXT-FIGURES 



By SIMON HENRY GAGE 

Professor of Histology and Embryology, Emeritus, Cornell University 



HENRY PHELPS GAGE, Ph.D. 



ITHACA, NEW YORK 

COMSTOCK PUBLISHING COMPANY 

1914 



G? 



COPYRIGHT 1914 

COMSTOCK PUBLISHING CO., 

ITHACA, N. Y. 




TO 

JACOB GOULD SCHURMAN 

IN GRATEFUL RECOGNITION OF HIS ABLE AND 
DEVOTED SERVICE TO CORNELL UNIVERSITY, OF 
HIS BREADTH OF SYMPATHY FOR ALL ART AND 
ALL SCIENCE, AND OF THE ENCOURAGEMENT 
AND SUPPORT WHICH HAVE MADE THE PRESENT 
WORK POSSIBLE, THIS VOLUME IS DEDICATED. 



PREFACE 

OUR aim in the preparation of this work on Optic Projection 
has been to explain the underlying principles on which the 
art depends, and to give such simple and explicit directions 
that any intelligent person can succeed in all the fields of projec- 
tion; and our hope is that the book will serve to make more 
general this graphic art by means of which many persons can be 
appealed to at the same time and in the most striking manner. 
Furthermore we believe that this art has great, undeveloped 
possibilities for giving pleasure, arousing interest and kindling 
enthusiasm, in that it provides for the rapid demonstration of 
maps, diagrams and pictures of all kinds, the structure and develop- 
ment of animals and plants, many of the actual phenomena of 
physics and chemistry, and finally scenes from nature and from life, 
even with their natural motions and colors. 

The authors have received most generous aid from many indi- 
viduals and many manufacturers; and most loyal service from 
those who have helped to put the book in its present actual form. 

Manufacturers have not only answered our numerous questions, 
but have put at our disposal valuable apparatus for experiment. 
They have also loaned us electrotypes of their apparatus. 

We feel especially indebted to the Department of Physics of 
Cornell University for the help given by different members of the 
staff, and for the use of a research room and apparatus for the 
numerous photometric and other determinations required. Pro- 
fessor George S. Molcr of that department read over the manu- 
script, and gave us many valuable hints derived from his experience 
of over 40 years with all kinds of projection apparatus. 

While we have both joined in the preparation of the entire 
work, each holds himself especially responsible for certain chapters 
as follows: 

The senior author for 10 chapters (I-V, VII-X and XII). 

The junior author for 5 chapters (VI, XI, XIII-XV). 

SIMON HENRY GAGE, 
October 4, 1914. HENRY PHELPS GAGE. 



CONTENTS 

Introduction pp. 3-7 

CHAPTER I 
Magic Lantern with Direct Current 

Fig. 1-38; 1-99, PP. 9-67 

Apparatus and material for Ch. I, i; Works of reference, 2; Magic 
Lanterns, 3-19; Perfection and brilliancy of the screen image, 20; 
Suggestions for the lecturer or demonstrator, 21-25; Suggestions to the 
operator, 26-41 ; Projection of horizontal objects with a vertical objec- 
tive, 42; Projection with multiple lanterns, 43-46; Moving slides for 
single lanterns, 47-49; Stereoscopic screen images, 50; Centering the 
parts of the magic lantern, 51-54; Correct separation of the parts, 55- 
56; Optical test for centering, 57-58; Centering the vertical objective, 
59-60; Troubles with the magic lantern, 61-93; Breaking of conden- 
ser lenses, 94-97; Examples of American magic lanterns, 99, fig. 32- 
38; Summary of Ch. I, 99,. 



CHAPTER II 
Magic Lantern with Alternating Current 

Fig. 39-40; 100-119 PP- 68-77 

Apparatus and material required, 100; Comparison of direct and alternat- 
ing current, 102-103; Installation with alternating current, 104-113; 
Use of the magic lantern with alternating current, 114-115; Troubles 
with alternating current lanterns, 116-118; Summary of Ch. II, 119. 



CHAPTER III 
Magic Lantern for Use on the House Electric Lighting System 

Fig. 41-55; 120-148 pp. 78-99 

Apparatus and material, 120; Magic lantern with small current for home 
and laboratory, 122-126; Arc lamps for the house circuit, 127-131: 
Turning the arc lamp on and off, 132-135; Magic lantern with mazda 
lamp, 136-139; Magic lanterns with Nernst lamp, 140-146; Troubles 
in Ch. Ill, 147; Summary of Ch. Ill, 148. 




Magic Lantern with the Lime Light 

Fig. 56-63; 150-186 pp. 100-118 

Apparatus and material, 150; Lime light with oxygen and hydrogen, 152- 
158; Management of the lime light lantern, 159-173; The lime light 
with oxygen and illuminating gas, 174-176; The lime light with oxygen 
and ether vapors, 177-179; Troubles with the lime light, 180-185; 
Summary of Chapter IV, 186. 



CHAPTER V 

Magic Lantern with a Petroleum Lamp, with Gas, Acetylene, and Alcohol 

Lamps 

Fig. 64-73; 190-224 pp. 119-137 

Apparatus and material, 190; Oil and gas lamps, 192-195; Magic lantern 
with kerosene lamp, 196-202; Lantern with mantle gas lamps, 203- 
207; Lantern with acetylene lamp, 208-213; Lantern with alcohol 
lamp and mantle, 214-219; Troubles in Ch. V, 220-223; Summary 
of Ch. V, 224. 

CHAPTER VI 
Magic Lantern with Sunlight; Heliostats 

Fig. 74-87; 230-265 pp. 138-165 

Apparatus and material for Ch. VI, 230; Light from the sun, and 
heliostats, 232-233; Installation and use of hcliostats, 234-238, 239- 
248; Heliostats for the southern hemisphere, 249-255; Condenser for 
sunlight, 256-258; Conduct of an exhibition with sunlight, 259-262; 
Troubles with sunlight, 263-264; Summary of Ch. VI, 265. 



CHAPTER VII 
Projection of Images of Opaque Objects 

Fig. 88-1 ii ; 270-297 pp. 166-199 

Apparatus and material for Ch. VII, 270; Images of opaque objects, 272; 
Comparison of the projection of opaque and transparent objects, 273- 
280; Combined projections, 281-282; Opaque projection demonstra- 
tions, 283-292 ; Erect images with opaque projection, 293-295 ; Troubles 
with opaque projections, 296; Summary of Ch. VII, 297. 



CONTENTS vn 

CHAPTER VIII 
Preparation of Lantern Slides 

Fig, 112-120; 310-340 pp. 200-220 

Apparatus and material for Ch. VIII, 310; Sizes of lantern slides, and 
necessary condensers, 312-314; Making lantern slides direct, 315-324; 
Photographic lantern slides, marking, mounting and coloring them, 325- 
337; Storing lantern slides, 338; Troubles in making lantern slides, 
339; Summary of Ch. VIII, 340. 

CHAPTER IX 
The Projection Microscope 

Fig. 121-178; 350-44 1 pp. 221-318 

Apparatus and references, 350-351; General on micro-projection, 352- 
354; Objectives, amplifiers, oculars, visibility of objects, diopter, 355- 
359; Room and screen, 360; Arc lamp and wiring, fine adjustment, con- 
denser, water-cell stage, mechanical stage, 361-369; Blackening appara- 
tus, 370-371 ; Hoods for objectives and shield for stray light, 372-373; 
Centering the projection microscope, 374-376; Table of candle power, 
377! Use of the projection microscope, 379-390; Magnification of 
screen images, 391-392; Projection with an ordinary microscope, 393- 
396; Projection of horizontal objects with a vertical microscope, 397; 
Sample objects for micro-projection, 399; Conduct of an exhibition, 
400; Demonstration with high powers, 401-411; Alternating 
current for micro-projection, 412-416; Micro-projection with the house- 
current, 417-418; Micro-projection with sunlight, 419-421; Micro- 
projection with lime light, 422-423; Home-made projection apparatus, 
424-431; Combined micro-projection and lantern slide projection, 
432; Projection microscopes on the market, 433-434; Troubles in 
micro-projection, 435-440; Summary of Ch. IX, 441. 

CHAPTER X 
Drawing and Photography with Projection Apparatus 

Fig. 179-220; 450-549 pp. 3 ! 9-389 

Apparatus and material, 450; Drawing with projection apparatus, general, 
452; Room for drawing, 453-455; Projection apparatus for draw- 
ing. 456-460; Light for drawing, 461-463; Drawing with the magic 
lantern, 464-468; Drawing with the reflecting lantern, 469-470; 
Drawing with a photographic camera, 471; Drawing with the projec- 
tion microscope, 472-485; Drawing with the house current, 486-491; 
Microscope to use with the house current, 492-503; Avoidance of heat, 



via OPTIC PROJECTION 

504; Stray light, 505-506; Magnification of drawings, 507-508; 
509-510; Drawings for models, 511; Erect images in the drawings, 
512-516; 517-526; Drawings for publication with projection appara- 
tus, 527-530; Drawings and their lettering, 531; Photography with 
projection apparatus, 532-547; Troubles met in Ch. X, 548; Sum 
mary of Ch. X, 549. 

CHAPTER XI 
Moving Pictures 

Fig. 221-236; 550-599 .. .pp. 390-438 

Apparatus and material, 550; Introduction, 552; Auditorium, screen 
and operating room, 553-557; Current, lamps and moving picture 
machine, 558-574; Installation of a moving picture outfit, 575; 
Optics of moving picture projection, 576-578; Magic lantern with the 
moving picture machine, 579; Management of the lamp, moving pic- 
ture machine mechanism, 580-589; flicker, 590-592; General precau- 
tions, 593; Splicing films, 594; Winding and rewinding, 595; 
Danger of fire, 596; Conduct of an exhibition, 597; Home projectors 
and advertising magic lanterns, 598; Troubles with moving pictures, 
599; Summary of Ch. XI, 599 1 . 

CHAPTER XII 
Projection Rooms and Screens 

Fig. 237-251 ; 600-642 pp. 439-473 

Apparatus and material for Ch. XII, 600; Suitable room for projection, and 
its lighting, 602-611; Position of the projection apparatus in the room, 
612-620; Screen for the image, 621-628; 629-632; Size of screens and 
screen images, 633-640; Troubles with rooms and screens, 641; 
Summary of Ch. XII, 642. 

CHAPTER XIII 

Electric Currents and their Measurement; Arc Lamps, Wiring and Control; 
Candle-Power of Arc Lamps for Projection 

Fig. 252-308; 650-782 pp. 474-571 

Apparatus and material for Ch. XIII, 650; Electric currents, kinds and 
comparison, 652-653; Electric units, 654-661; Electric measure- 
ments and apparatus, 662-671 ; 672-674; Power factor, cycle, frequency, 
675-677; Special dynamo for arc lamps, 678-680; Current rectifiers, 
681-683; 135 and 25 cycle currents for projection, 684-685; Wiring 
for arc lamp from dynamo back to dynamo, 686; Amperages for 
different purposes, short circuit, ground, insulation of wires, 687-690; 



CONTENTS ix 

Regulations for wiring, 691-692; 693-700; Polarity tests for direct 
current, 701-703; Wiring the three-wire automatic lamp, 704; Wiring 
for alternating current, 705-710; Switches, circuit breakers and fuses 
711-722; Resistors or rheostats, 723-735; Reactors, inductors, 
choke-coils, etc., 736-738; Transformer, 739; The electric arc, 740- 
743; The use of ballast (rheostats, etc.), 744 748; The arc lamp, light 
and heat from, 749-752 ; Carbons and their position, 753 ; Table show- 
ing size and wear of carbons, 753a; Candle power of arc lamps, 754- 
762; Candle power, and energy required, 763-768; Distribution of light 
intensity in different directions, 769-771; Intrinsic brilliancy of the 
crater, 773; Visible and invisible radiation, 774; Radiant efficiency 
of arc lamps, 775-776; Energy required for moving picture projection, 
779-78i ; Effect of opacity in the film, 782. 

CHAPTER XIV 
Optics of Projection 

Fig- 309-349; 790-865 pp. 572-620 

Reflection and refraction, 792-801; Lenses, 802-808; Spherical and 
chromatic aberration in lenses, 809-810; Image formation with the 
magic lantern, 811-817; Focus of Condenser and objective, 818; 
Types of condensers, 819-821; Image formation with moving pictures, 
822-832; Image formation with the projection microscope, 833-838; 
Light losses, 839-843; Energy losses, 844-854; Effect of aperture, 
855-856; Brightness of the screen image, 857; Microscopic image 
and aperture, 858-863; Koehler method of illumination, 864-865. 

CHAPTER XV 
Uses of Projection in Physics; Normal and Defective Vision 

Fig. 350-402; 875-932 pp. 621-672 

Apparatus and material for Ch. XV, 875; Introduction, 877-878; Experi- 
ments with polarized light, 879-884; Projection of spectra, 885-900; 
Absorption spectra, 901-902; Emission spectra, 903-905; Ultra- 
violet light, photography, 906-908; Abbe diffraction theory, 909-911; 
Dark ground illumination, striae, 912-915; Normal vision and eye 
defects, 916-932. 

Appendix. Brief Historical Statement on the Origin and Develop- 
ment of Projection Apparatus 673 

Projection Apparatus and Accessories in the Open Market; Manu- 
facturers 688 

Bibliography on Projectior 693 

Index of names and subjects 705 



INTRODUCTION 

IN THE following pages, Projection Apparatus of various forms 
and with various sources of light have been considered from 
a three-fold standpoint: 

(1) The standpoint of the actual user of the apparatus. 

(2) The standpoint of the manufacturer. 

(3 ) The standpoint of the student for whom an understanding 

of the principles involved is of fundamental importance. 

From the first and second standpoints simple "rule of thumb" 
would answer, and in many cases has answered to bring about 
fairly good results. For example, the toy magic lanterns so much 
in evidence at Christmas time, are almost exact copies of the first 
magic lantern shown by Walgenstein in 1665. The only striking 
difference is that instead of a candle or lamp without a chimney 
such as he used, there is now a small petroleum lamp with a glass 
chimney. 

But for adapting projection apparatus to new conditions and 
applying it to new uses with the greatest efficiency, the user and 
the manufacturer must comprehend the fundamental optical and 
mechanical principles involved. In a word, to make good projec- 
tion apparatus and to produce good projection in the different 
fields, the manufacturer and the user must know the principles, 
and then they must build and must use the apparatus in accordance 
with those principles. 

Besides the optical and mechanical principles involved in the 
apparatus, it seems to the authors that the physiology of vision 
should have prime consideration, because, after all, it is not only 
the possibility of producing a brilliant screen image that must be 
thought of, but also the possibility that the observer get a satis- 
factory impression of that image. With the magic lantern and 
arc light it is very easy to get screen images as brilliant as daylight 
scenes in nature. These brilliant images are best seen when the 
eyes of the observers are adapted to daylight vision. If now, as is 



4 INTRODUCTION 

possible with modern combined apparatus, the brilliant screen 
image of the transparency is replaced by a relatively dim image 
projected by the opaque lantern, it will appear exceedingly dim 
until the eyes can be adjusted to twilight vision. If the operation 
is reversed after the eyes are adapted to dim light, the brilliant 
screen image of the transparency will dazzle the eyes. 

It is then, not only the dead machine that must be considered, 
but also the living machine the eye. It is for the eye that all the 
work is done, and perfection can be gained only by understanding 
the workings of the two machines, and adapting the dead machine 
to the physiologic laws governing the living machine. 

Our aim in writing this book then has been to show how good 
results can be most easily and certainly obtained in all the forms of 
projection by obeying the laws of physiology as well as those of 
optics and mechanics. 

Naturally, most users of projection apparatus will employ that 
which is regularly manufactured, but in many institutions not all 
of the desired apparatus can be afforded. Furthermore, every one 
who is to do any special work in projection must be capable ot 
combining and adapting apparatus for those special needs. Hence, 
we have indicated how home-made apparatus can be got up, and 
how apparatus designed for one purpose can be utilized for other 
purposes. We have done this for two reasons, first, because we 
feel sure that a great gain in efficiency can be made in teaching by 
the use of the magic lantern, the projection microscope and other 
forms of projection apparatus, and secondly, because the con- 
struction or adaptation of projection apparatus gives one an 
intimate and working knowledge which more than pays for all the 
time and trouble. 

In examining the apparatus of many different makers we have 
been impressed with the general excellence of the apparatus and 
also with certain general defects. 

The defects seem to us almost wholly due to the fact that the 
manufacturers of apparatus and the users of the same are not 
intimately enough associated, and, therefore, are not so mutually 
helpful as seems desirable. 



INTRODUCTION 5 

The manufacturer naturally advertises the possibilities of his 
apparatus as if he expected it to be used under the most favorable 
conditions, and operated by men skilled in the use of optical instru- 
ments, and the results to be judged by persons of experience who 
do not expect the impossible. 

For example, if one reads the statements concerning the projec- 
tion of pictures in books, photographs, postal cards and actual 
objects, the impression would be very strong that the screen pic- 
tures so produced were every bit as satisfactory as those of lantern 
slides, and just as easily produced. In speaking with many 
individuals we have found the belief is very general that with the 
new apparatus nothing is simpler than to get good screen images 
of objects, pictures, etc., with all their natural colors, and that the 
expense of lantern slides can be wholly done away with. But we 
have yet to find the actual user of such apparatus who found his 
sanguine expectations fully realized. 

Modern opaque projection is marvelous in its accomplishments, 
but what is gained in the use of actual objects, books, etc., is lost in 
the relative dimness of the screen image, in the expense and diffi- 
culty of managing the apparatus, and in the large electric current 
needed to give even tolerable screen images. 

Judging from our observations the manufacturers have not fully 
realized the lack of optical and mechanical knowledge and instinct 
in many users of projection apparatus. Naturally, the user of the 
apparatus wants results, and he wants the apparatus to give the 
results without trouble. 

Perhaps the most striking, as also it seems to us the most easily 
obviated defect, is, that with many parts of the apparatus, it is just 
as possible to insert them in the wrong position as in the right 
position. For example, in most of the apparatus we have examined 
the condenser is so mounted that it can be put with either end 
facing the arc lamp. So with many other parts, they can be put in 
a wrong position just as easily as in a right position. 

In our opinion there are five fundamental rules in the production 
of projection apparatus that the manufacturers should follow: 



6 INTRODUCTION 

1. The optical parts should be arranged on one longitudinal 

axis and fixed in that position, except that the projection 
objective must be movable along the axis for focusing. 

2. The radiant or source of light should be adjustable in every 

direction to insure proper centering of the light along the 
optic axis, and to insure the proper relative position of the 
source of light and the condenser. 

3 . The object carrier for lantern slides is preferably fixed in one 

position; but the stage of the microscope and the object 
holder of most other kinds of apparatus should be movable 
along the longitudinal axis so that the object can be put in 
the cone of light where it will be fully and most brilliantly 
lighted. 

4. The parts should be constructed so that either (a) it makes 

no difference how they arc placed or (b) so that they can- 
not be put together wrong. (See footnotes to 4-5, p. 7). 

5. Every part of the apparatus should be dull black to avoid 

reflections. 

Of course for experimental apparatus the more adjustable each 
part is the greater are its possibilities, but for apparatus to use for 
definite purposes we believe that no unnecessary adjustments 
should be possible. 

The custom followed by many manufacturers of sending an 
illustrated pamphlet giving instructions for installing and using 
their apparatus, is wholly commendable. In addition it would be 
advisable in some cases to attach tags to the different parts, stating 
their purpose and connections (fig. 45). 

All of the apparatus and all of the experiments discussed in this 
book have been personally tested or observed by us to make sure 
that they will work; and we have tried to give directions and 
methods which arc intelligible, and which will most easily produce 
the desired results. 

Finally, the authors of this book most earnestly advise any one 
who is to use projection apparatus to go to some place where the 
facilities arc abundant, and where there is someone skillful in 
using them. This will give him a standard of what can be accom- 



INTRODUCTION 7 

plished and what can reasonably be expected. The learner will 
find that in such a place the apparatus, the room, the screen and 
the light are all adapted to the purpose to be served. 

Good projection, like any other skilled operation, requires 
knowledge, facilities and experience. 



There is a very trenchant expression used in shops and in laboratories which 
seems to us to cover the ground. It is: "Fool Proof." 

From the testimony of many who are especially skilled in machinery and in 
the use of apparatus, and from our own personal experience, the "fool proof" 
construction of apparatus is not only necessary for the careless and unskilled, 
but much appreciated by the most skilled and careful. When one is absorbed 
in the principles and complexities which some experiment is meant to elucidate, 
it is a great advantage to have the apparatus which is to be used so constructed 
that it will go together in the right way with the least conscious effort on the 
part of the user. The user ought not to be compelled to make a special study 
of the apparatus every time it is assembled. It is the business of the manu- 
facturer to put thought into the construction of the apparatus, and it is the 
user's business to work out problems with it. 

From time immemorial it has been the habit of mankind to make tools, 
implements and more elaborate apparatus with smooth and glistening surfaces, 
bright colors often being added to heighten the effect. The microscope and 
other optical apparatus naturally followed the fashion. 

While to many workers in optics there early came the fundamental apprecia- 
tion that the clearest images were possible only when absolutely no light 
reached the eye except from the image field, still polished brass and nickel 
finish persisted, and the dazzling reflections when bright lights were used, often 
overwhelmed the image which it was the sole purpose of the apparatus to make 
visible. 

During the last few years the knowledge of the best conditions for clear 
images has asserted itself more and more, and the mirror surfaces of optical 
apparatus have gradually disappeared. At first the dull black apparatus was 
prepared only for the few who could demand and pay for a special finish. The 
advantage of the dull finish of optical apparatus is so apparent when once seen 
and used that now it is becoming very common. 

The great advantage of such dull black, non-reflecting surfaces for the out- 
side as well as for the inside of optical instruments became apparent to the 
senior author by the accident of a laboratory fire (1900) during which the 
lacquer of his best microscope was blackened by the dense smoke. 

The ordinary point of view ten to fifteen years ago that optical apparatus 
should of course have a 'bright brass or nickel finish is well illustrated by this 
incident: The senior author was having, by special contract, a microscope 
with all its accessories made dull black. A visitor, interested in optical goods, 
going through the factory noticed this lone, black microscope among the 
brilliant array and asked: "When are you going to bury that one?" 



CHAPTER I 

THE MAGIC LANTERN WITH DIRECT CURRENT ARC 
LAMP AND ITS USE. 

1. Apparatus and Material for Chapter I: 

Suitable projection room with screen (Ch. XII) ; Magic lantern 
( 3~ I 9) I Arc lamp, automatic or hand-feed, with fine adjustments, 
lamp-house and wiring for current up to 25 amperes (fig. 3); 
Cored carbons adapted to the current ( 7 53 a) ; Rheostat; Lantern 
table (Ch. XII); Double-pole, knife switch ( 8); Ammeter ( 7); 
Incandescent lamp or flashlight ( 14-15); Gloves with asbestos 
patches ( 27); Lantern slides; Opera-glasses ( 38); Testing 
incandescent lamp ( 6 1 , fig, 21); Fuses ; Extra condenser lenses to 
replace cracked ones ( 94) ; Screw driver and pliers. 

2. Historical Summary and Works of Reference: 

For a historical summary of the invention and use of the Magic 
Lantern, see the Appendix. 

The reader will find many good hints in the following works on 
Projection. For the full titles, see the Bibliography. 

R. C. Bayley. Modern Magic Lanterns and their Management. 

H. Fourtier. La Pratique des Projections. 

Hassack and Rosenberg. Die Projektionsapparate. 

T. C. Hepworth. The Book of the Lantern. 

R. Neuhauss. Lehrbuch der Projektion. 

C. G. Norton. The Lantern and How to Use It. 

F. P. Wimmer. Praxis der Makro und Mikro-Projektion. 

Lewis Wright. Optical Projection. 

The latest information and many useful hints may be found in 
the catalogues of the manufacturers (see Appendix). 

MAGIC LANTERN 

3. The Magic Lantern as the standard for projection appara- 
tus. The magic lantern may be taken as the standard example of 
projection apparatus, for it is in the most common use and is the 
simplest instrument for image projection. 



10 



MAGIC LANTERN WITH DIRECT CURRENT [Cfl. I 



Condenser 



Arc La 




KS 



FIG. i. SIMPLEST FORM OF MODERN MAGIC LANTERN WITH ARC 

LAMP 

It consists of an arc lamp with suitable connections to the current supply, 
a rheostat and a table switch; a double condenser, lantern-slide holder and 
projection objective. 

Arc Lamp This is a mechanism for holding and feeding the carbons. 

h c Horizontal (upper) and 

v c Vertical (lower) carbons. 

5 5 Set screws for holding the carbons in place. 

In In Insulation between the carbon holders, and the rest of the lamp to 
prevent a short circuit. 

/ 5 Feeding mechanism for moving the carbons. 

d Clamp for fixing the lamp in any position on its vertical support. 

SW Supply wires to the lamp socket or wall receptacle. 

So Lamp socket. 

K Key of the socket switch. 

5 P Separable attachment plug. 

L W Supply wires from the cap of the attachment plug to the table switch. 

K S Double-pole knife switch on the table for turning the current off and 
on the arc lamp. 

Rheostat for controlling the current. It is inserted in one wire. 

Condenser In this simple form it is composed of two plano-convex lenses 
with the convexities facing each other. 

I 2 The two elements of the condenser. 

L S Lantern slide close to the plane face of the 2d condenser lens. 

Axis Axis The straight line passing from the source of light along the 
optic axis of the condenser and the objective to the image screen. 

Objective The projection objective for giving a clear image of the lantern 
slide on the screen. 

c The center of the objective where the rays from the condenser should 
cross. 

Image Screen The white screen upon which the image of the lantern slide 
is projected by the objective. 



If the principles governing the magic lantern are mastered, and 
one gains skill in handling it, the more difficult forms of projection 
will offer no great obstacles. 



CH. I] MAGIC LANTERN WITH DIRECT CURRENT 1 1 

4. Standard source of light. With all forms of present day 
projection the direct current arc light is taken as the standard 
because, next to the sun, it is the most perfect light source available. 
In many places it is to be had during the entire twenty-four hours, 
and is the safest and most easily managed light capable of furnish- 
ing sufficient illumination for use with all kinds of apparatus, from 
the simplest magic lantern to the moving picture machine and the 
compound microscope. 

MAGIC LANTERN WITH DIRECT CURRENT ARC LIGHT 

Except the projection table, the room and screen, (for which see 
424 and Ch. XII,) the essential elements of a magic lantern and 
their arrangement are shown in fig. i, 2, 3. They are as follows: 

5. Wires for the electric current. There must be two wires 
for carrying the current extending from the main line to the electric 
lamp. One wire, the positive ( + ), conveys the current to the 
upper carbon of the lamp, and the other, the negative ( ), conveys 
the current from the lower carbon back to the main line (fig. 1,2) 
(see also Ch. XIII). 

6. Rheostat. This device must be placed in the circuit along 
either the positive or the negative wire, it makes no difference 
which. In figures i and 2 it is placed in the negative wire. 
It serves as a balance, and [limits the amount of current pas- 
sing through the lamp ( 744-748). 

7. Ammeter. This indicates the amount of current flowing. 
It is not necessary, like the rheostat, but is very desirable, for with 
the information it gives, the operator can determine whether any 
defects in the brightness of the screen image are due to the lack of 
current, or whether something else is at fault (see Troubles. 
61-95.) 

The ammeter is placed along one wire the same as the rheostat 
(fig. i, 2). 

In case no ammeter is used the rheostat can be calibrated and 
marked when the apparatus is installed (see 729). 



12 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I 

8. Double-pole switch. It is important to have a double-pole 
switch near the lamp. By its means the operator can at any time 
turn the current on or off the lamp. When the switch is open no 
current can reach the lamp (fig. 1-3). 

9. Arc Lamp; automatic type. The lamp is needed to hold 
the carbons, and to provide a mechanism for moving them toward 
each other as they burn away (see 12). The lamp may be of the 
automatic type in which there is a magnetic release or motor for the 
mechanism, so that the carbons are brought nearer together when- 
ever the arc gets too long. If it is properly designed and con- 
structed, the lamp will burn continuously as long as the switch is 
closed, and the carbons last. There should also be a hand-feed 
mechanism in these arc lamps, so that slight modifications may be 
made by hand when necessary ; furthermore, there must be arrange- 
ments for moving one or both carbons separately to correct any 
irregularity in the wasting away of the carbons. 

10. Fine adjustments. There must be adjusting screws by 
means of which the lamp can be slightly raised or lowered, or moved 
to the right or to the left, to enable the operator to keep the crater 
of the positive carbon exactly in the axis. This is to compensate 
for the slight change in position of the crater as the carbons burn 
away (fig. 3). 

11. Arc lamp, hand-feed type. In this form of arc lamp the 
operator must work the mechanism by hand. The carbons usually 
have to be moved nearer together every four or five minutes. As 
with the automatic type, one or both carbons should be movable 
independently, and there should be fine adjustments ( 9, 10). 

12. Carbon Terminals. As a light source for projection, 
carbon terminals or electrodes are used in the arc lamp.. With a 
direct current the carbons burn away unequally, the upper, positive 
carbon, wasting about twice as fast as the lower, negative carbon. 
If the carbons are of equal size and quality, the mechanism of the 
lamp must move the upper carbon about twice as fast as the lower 
one. Some times a lamp with equal motion for the upper and lower 



CH. II 



MAGIC LATERN WITH DIRECT CURRENT 



Condenser 



H C 




FIG. 2. MAGIC LANTERN WITH TRIPLE CONDENSER AND 
WATER-CELL. 

H C, V C Horizontal or upper and vertical or lower carbon of an arc lamp. 
The upper carbon furnishes the light. 

D + C Supply wires for the electric current. The positive wire (+) goes 
to the upper carbon (H C), and the negative wire ( ) comes from the lower 
carbon (V C). The arrows indicate the direction of the electric current. 

F Fuses where the supply wires for the lamp connect with the main line. 

L Incandescent lamp with wire guard. It is connected with the supply 
wires before the table switch (S) and the resistor (R), hence it can be used 
while the arc lamp is running or when it is turned off (See also fig. 4). 

S Double-pole, knife switch for turning the current on or off the arc lamp. 

R Rheostat for controlling the current. It is inserted in one wire. 

A Ammeter to indicate the amount of current being used. It is inserted 
in one wire. 

Condenser This consists of a meniscus next the arc light, and of two plano- 
convex lenses with a water-cell between them. The lenses must be arranged 
as here indicated. 

W Water-cell placed between the plano-convex lenses of the condenser. 
It absorbs much of the radiant heat. 

L S Lantern slide close to the condenser. 

Axis Axis The straight line passing from the source of light along the 
optic axis of the condenser and the objective to the screen. 

Objective Projection objective serving to give a clear image of the lantern 
slide on the screen. 

C Center of the objective where the rays from the condenser should cross. 

Screen Image The image of the lantern slide formed by the objective on 
the white screen. 

carbons is used and the upper carbon is enough larger than the 
lower one, so that the two shorten at the same rate. 

In our experience it is more satisfactory to have both carbons 
with soft cores, but some advocate and use a large soft-cored carbon 
above and a smaller solid carbon below (fig. 299). 



U MAGIC LANTERN WITH DIRECT CURRENT [Cn. I 

13. Lamp-house. This is a metal box in which the arc lamp 
is enclosed. It should be of good size, and be well ventilated by 
means of openings at the bottom, and a flue at the top. There 
should be one or more large doors, so that the lamp can be reached 
for changing the carbons and making any necessary adjustments. 
Opposite the crater at the end of the positive carbon there should 
be a window about 2 to 3 cm. (2 in.) square so that the ends of the 
carbons can be observed when the lamp is burning without opening 
the door. This window should be provided with a combination of 
red and green, or red and blue glass, or with smoky mica or with 
deeply tinted glass so that the eyes will not be injured when look- 
ing at the crater (fig. 133, 147). 

14. Incandescent lamp. If experiments are to be made it is 
desirable to have an incandescent lamp with wire guard to use in 
connection with the lantern. It should have a flexible cord of 
sufficient length so that it can be carried to any desired position. 
This lamp must be connected with the supply wires before the 
rheostat is inserted ; then it will burn brightly while the arc lamp 
is going. By consulting fig. 2, it will be seen that the two wires for 
this lamp are connected one with each of the supply wires. That 
is the incandescent lamp is not connected with one wire like the 
rheostat and the ammeter but with both wires. 

15. Electric flash-light. An electric flash-light is a great 
convenience about a lantern; and is almost a necessity when an 
incandescent light (fig. 1,2) is absent. It should lock, so that it 
will burn continuously ; then carbons may be changed by its light 
and other corrections made. It is an absolutely safe light also. 

16. Incandescent lamp to burn when the arc lamp is turned 
off. To avoid the great darkness in the room when the arc lamp is 
turned out, it is advantageous to have an incandescent lamp con- 
nected with the line, as indicated in fig. 4. 

17. Condenser. --This collects the light from the arc lamp 
and directs it through the objective. In passing from the con- 
denser to the objective it passes through the lantern slide or other 
object whose image is to be projected (fig. 1,2 4). 



CH. I] 



MAGIC LANTERN WITH DIRECT CURRENT 




FIG. 3. ARC LAMP FOR PROJECTION, WITH WIRING, SWITCHES 
AND FUSES 

Supply Wires The conductors from the supply to the outlet box. 

Outlet Box. An iron box receiving the supply wires at one end and giving 
exit to them from the other. 

Fuses & Switch Two cartridge fuses in the circuit and a double-pole knife 
switch beyond the fuses. The fuses are present to avoid accident in case of a 
short circuit and the switch to turn the current on or off as desired. 

P W R Polarized wall receptacle. This is composed of two parts as 
shown, the part on the wall to receive the supply wires from the outlet box, 
and the cap to connect with the table switch. The metal connections of the 
cap with the receptacle arc in planes at right angles so that the cap can be put 
in place only in one way, hence the polarity is always the same. 

Arc Supply The wires connecting the cap of the wall receptacle and the 
table switch. 

Switch The double-pole, knife switch for turning the current on and off the 
arc lamp. 

Wi The wire extending from the switch to the upper carbon of the arc 
lamp. 

W2 The wire extending from the switch to the rheostat. 

If j The wire extending from the rheostat to the lower carbon of the arc 
lamp. 



1 6 MAGIC LANTERN WITH DIRECT CURRENT [CH. I 

Rheostat This is for controlling the current. It is inserted in one -wire. 

Arc Lamp The mechanism for holding and feeding the carbons. 

F S Feeding screws for moving the carbons closer together or farther 
apart. The carbons can be moved separately or both at once. 

V A Fine adjustment screw for moving the carbons up or down. 

L A Fine adjustment screw for moving the carbons to the right or left. 
in in Insulation between the carbon holders and the rest of the lamp. This 
is to prevent the current from leaving the carbons and making a short circuit 
through the metal part of the lamp. 

.s ^ Set screws for holding the carbons in place. 

Lamp-House The metal box enclosing the arc lamp. The feeding screws 
(F S) and the line adjustments (V A, L A) should project through the wall of 
the lamp-house. 

Condenser A condenser composed of three lenses with a water-cell in the 
parallel beam between the plano-convex lenses. 

/ The first element of the triple condenser is composed of a meniscus lens 
next the arc lamp, and a plano-convex lens next the water-cell. 

2 The second element of this condenser is a plano-convex lens. The con- 
vex surfaces of the plano-convex lenses face each other as in the double con- 
denser (fig. i). 

Block i. The block supporting the arc lamp. It is movable back and forth 
along the track on the base-board. The socket and set screws permit the 
adjustment of the lamp. 

Block 2. The block holding the condenser. It is movable along the track 
cm the base-board. The socket and set screw (S) enable one to adjust the 
position of the condenser. 

Base Board The board on which all the parts of the projection apparatus 
rest (see fig. 158-159). 

The condenser is of two or of three lenses. If of three lenses the 
first lens, which is nearest the arc lamp, is usually of meniscus form, 
with the concavity next the lamp. The second lens is a plano- 
convex, as is also the third (fig. 2). If the condenser is of two 
lenses both are usually plano-convex with the convex surfaces fac- 
ing each other and the plane faces looking toward the radiant and 
toward the lantern slide (fig. i). 

The two condensers appear alike in form and relation of the 
lenses except that in the three-lens type a meniscus has been added. 

In the three-lens type the meniscus and first plano-convex 
together render the diverging light from the lamp parallel, and the 
third lens or second element renders this parallel beam converging, 
bringing it to a focus at the center of the projection objective when 
the condenser and objective are properly proportioned to each 
other (fig. 1-2). 

AVith the two-lens condenser the usual practice is to bring the 
condenser closer to the lam]) than the focal length of the first lens. 



CH. I] MAGIC LANTERN WITH DIRECT CURRENT 




FIG. 4. MAGIC LANTERN WITH INCANDESCENT LAMP IN THE CIRCUIT 
AFTER THE RHEOSTAT (Compare fig. 2). 

W W Supply wires. 

F Fuses in the supply wires (see fig. 3). 

R R Rheostat for controlling the current. 

A Ammeter for indicating the amount of current. 

p p The two binding posts of the knife switch. The two wires of the 
incandescent lamp are connected at these points. 

b s The incandescent bulb and the key switch of the lamp socket. From 
the connections of the supply wires to the incandescent lamp it will shine 
whenever the socket key is closed whether the knife switch to the arc lamp is 
opened or closed. When the arc lamp is burning the incandescent lamp will 
be very dim and when the arc lamp is out it will shine with full brilliance. 

S The table, knife switch. 

L The source of light. 

The +'s, 's and arrows indicate the polarity and course of the electric 
current. 

Condenser A two-lens condenser with water-cell (W). 

L S Lantern slide. 

Axis The principal optic axis of the condenser and of the objective. 

Objective The objective for projecting an image of the lantern slide upon a 
screen. 

Screen Image. The image projected on the screen by the objective. 

This gives a somewhat diverging beam between the two lenses. 
The second lens brings this diverging beam to a focus beyond its 
own principal focus. 

This condenser is sometimes placed so that the crater of the arc 
lamp is at the principal focus of the first lens and the center of the 
projection objective at the focus of the second lens, as in fig. 2. 

Whatever the form of the condenser, the lenses must be so 
mounted that there is freedom for expansion; and they must be 
so arranged that the proper lens is next the radiant (see fig. 2, 3, 
36 B). 



1 8 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I 

18. Water-cell. This is a vessel of water with parallel, glass 
sides, placed in the beam of light from the lamp, before the light 
reaches the lantern slide or other object. The water-cell absorbs 
most of the radiant heat from the lamp and thus protects the 
objects from over-heating (fig. 2-3). 

The water-cell is especially needed for opaque lantern slides like 
those of dark scenes or colored slides made by the Autochrome 
process. It sometimes happens that in an exhibition as many as 
10 to 30 per cent, of the slides are cracked by the heat, if no water- 
cell is used. 

Unfortunately the water-cell is oftener absent than present in 
magic lanterns. (For a further discussion of the avoidance of heat 
see 364, 854). 

19. Projection objective. This forms an image of the lan- 
tern slide upon the screen. If the instrument is in proper adjust- 
ment the objective will transmit to the screen the rays of light from 
the condenser which pass through the lantern slide or other semi- 
transparent object. These rays reflected from the screen to the 
eye give rise to a picture with all the gradations of light and shade 
and color of the lantern slide or other object used (see fig. 1,2, and 
811). 

PERFECTION AND BRILLIANCY OF THE SCREEN IMAGE 

20. The quality of the screen image depends upon : 

1 . The accurate centering along one axis of the source of light, 

the condenser, and the projection objective (fig. 1,2). 

2. The amount and intensity of the light used. 

3. The excellence of the condenser. 

4. The aperture and perfection of the objective. 

5. The proper proportion of the objective and the condenser to 

each other and to the size of the room. (See fig. i, 2, 
634-636). 

6. The perfection and transparency of the lantern slides or 

other objects imaged on the screen. 

7. The accuracy of the focus of the image on the screen. 

8. The reflecting qualities of the screen (sec 621). 



CH. I] MAGIC LANTERN WITH DIRECT CURRENT 19 

9. The darkness of the projection room (see 608). 
10. The proper adjustment of the eyes of the spectators to 
either daylight or twilight vision ( 281). 

USE OF A MAGIC LANTERN FOR EXHIBITIONS AND FOR 
DEMONSTRATIONS 

SUGGESTIONS TO THE LECTURER OR DEMONSTRATOR! 

21. Order of the lantern slides. The lecturer or demon- 
strator should have his slides in the exact order in which they are 
to be shown. They should not only be in the exact order of exhibi 
tion, but they should all be in the same relative position so that 
the operator can insert them correctly without the trouble of 
looking at them individually. 

22. Duplication of lantern slides. It frequently happens 
that the same slide, for example, of a map or some other general 
subject, should be shown at two or more stages of a lecture. There 
is always difficulty in doing this unless the operator is carefully 
instructed, and the slide is marked to be repeated, and a slip of 
paper inserted in the pile of slides at the proper level. With a 
small audience, and for an informal talk the difficulty is, perhaps, 
not great; but for a large audience and anything like a formal 
presentation, the repetition of the same slide almost always causes 
confusion and delay. 

To avoid this confusion, one can have duplicate lantern slides. 
Then the slides can be put exactly in order, and no confusion is 
possible. 

If a person has ever exhibited lantern slides for a friend, and one 
or more of the slides had to be shown two or three times, he can 
understand the troubles of the operator when the same slide must 
be shown more than once, and will agree that it is better to have 
the slide duplicated. 

23. Marking or "spotting" lantern slides. In order that 
lantern slides may be inserted in the carrier by the operator 
correctly, and without hesitation or worry, the slides must be 
marked or "spotted" in some conspicuous way (fig. 7, 8, 13). 



20 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I 

If the slides are not marked, and the correct position must be 
determined for each individual slide during the exhibition, even 
the most expert operator is liable to make mistakes, especially 
when the slides are shown rapidly. 

24. Inspection of the room and lantern by the lecturer. It 

is highly desirable that the lecturer make himself acquainted with 
the room in which he is to speak, and inspect the lantern himself 
before the lecture hour. If the operator is with him it gives 
opportunity to establish pleasant relations, and to stimulate the 
operator to make the best exhibition possible. It also gives oppor- 
tunity and time to make any slight changes necessary to insure a 
good exhibition. Foresight is always more satisfactory in its 
results than hindsight. 

25. Directions for the operator. The lecturer should in- 
struct the operator how he wishes the slides shown. There must 
be some signal for changing the slides. Preferably the signaling 
device is some form of electric signal on the operator's table, then 
he can see or hear it, but the audience will not be distracted by it, 
as when the lecturer has to speak to the operator, or hammer on 
the floor with the pointer, etc. (For signaling devices see the list 
of apparatus in the appendix). 

The lecturer should direct the operator to light the lantern 
before the room lights are extinguished, and give ample warning. 
The operator should also be told to leave the lantern burning 
until the room lights are turned on. 

SUGGESTIONS TO THE OPERATOR 

26. Testing the lantern. Before every exhibition or demon- 
stration the operator should make sure that the lantern is in good 
working order. This is only fair to the speaker who depends upon 
his illustrations which he has taken so much trouble and expense to 
prepare. If the slides are not well shown it injures the effect of the 
lecture or demonstration and makes it difficult or impossible for 
the speaker to make clear the subject he is treating. It also dis- 
quiets the audience; and should make the operator uncomfortable. 



CH. I] MAGIC LANTERN WITH DIRECT CURRENT 21 

In testing the lantern the following points should be especially 
looked to : 

(A) That there is voltage in the supply line. This is easily 
determined by turning on the incandescent lamp (fig. 2), or by 
trying to light the arc lamp. 

(B) That the arc lamp is in working order and has carbons long 
enough to last during the exhibition. By closing the switch and 
bringing the carbons in contact and slightly separating them the 
arc light should be established almost instantly (see also 30). 
It takes a certain amount of experience to tell whether the carbons 
are long enough to last during the exhibition. If there is any 
doubt, put a new pair in position. 

From the high temperature of the carbons, and the lamp gener- 
ally, after the current has been on some time, it is not easy to put 
in new carbons in the midst of a demonstration. It also makes an 
embarrassing break in the exercises (see 27). 

27. Gloves with asbestos patches. In spite of all precau- 
tions it is sometimes necessary to work about the arc lamp after it 
has been running, and is therefore very hot. By the use of suitable 
pliers or tongs one can usually manage to do the things necessary ; 
but for certainty and rapidity one always needs to be able to use 
the hands directly. This is rendered possible by the use of gloves 
with asbestos patches in the places which come in direct contact 
with the hot metal or carbons. The gauntlet form of gloves is best 
for then the wrists also are protected. 

The asbestos patches may be of asbestos cloth, or preferably of 
quilted asbestos paper. The asbestos cloth is very thick and 
clumsy. The asbestos paper of about half a millimeter thickness 
(Vso in.) quilted between thin cotton or linen cloth answers well. 
The quilting stitches should be long and extend obliquely in two 
directions (fig. 5). The object of the quilting is to overcome the 
weakness and easy tearing of the asbestos paper. 

For most work a patch on the thumb and index finger is sufficient 
but as it is often convenient to grasp a hot carbon between the 
index and middle finger, it is well to have a patch on the middle 
finger also (fig. 5). 



22 MAGIC LANTERN WITH DIRECT CURRENT ICH I 




FIG. 5. GLOVES WITH ASBESTOS PATCHES, PALM SIDE UP. 

Left glove, p. i. m. The pollex or thumb (/>), the index or fore finger 
(i), and the medius or middle finger (m), have the patches on the palmar sur- 
face and sides. 

c Carbon held pincer-like between the index and medius. 

Right glove. The asbestos patches are as in the left. Above the correspond- 
ing digits (i, 2, 3) are patterns of suitable patches drawn to the same scale as 
the gloves. 



With the hands protected by such gloves, one can grasp the hot 
carbons within two or three centimeters (i in.) of the hot tips with 
entire safety. The asbestos being a non-conductor of electricity 
as well as of heat, makes it safe also to work about the lantern when 
the current is on ( 2 7 a). 



27a. Old leather gloves answer very well if one does not wish to sacri- 
fice a new pair. New cloth gloves with gauntlets can be had for 20 cents. 
These answer fairly, but are not so good as the leather gloves, and there is 
no danger of the leather gloves being motheaten or catching fire. It is easier 
to sew the patches on the cloth gloves, however. 

Asbestos mittens are to be had of dealers in chemicals and chemical 
apparatus. They are of asbestos cloth but are so thick and clumsy that they 
are not adapted for working about the lantern. 



CH. I] 



MAGIC LANTERN WITH DIRECT CURRENT 



28. See if the lantern is centered. Make sure that the 
different elements of the lantern are centered along one longitudinal 
axis (fig. 1,2). Then and then only will a perfect screen image be 
produced. If the apparatus was installed correctly in the begin- 
ning the only part liable to be out of line is the crater of the positive 
carbon. In burning the carbons frequently so wear away that the 
crater is at one side of the axis. Slight decentering of the crater 
can be easily corrected by using the fine adjustment designed for 
the purpose ( 10, fig. 3, see also Troubles 79). 

29. Slide-carrier. Be sure that the slide-carrier works 
properly and easily. The "push-through" form (fig. 6), is very 
convenient, for while one slide is on exhibition the one previously 
shown can be removed and another put in place, and it can be 
instantly put in front of the condenser when the lecturer signals. 





3 






3 










S2 






S1 ~ -' 










eujiJi-j eoiBeyg euj8}e~| 
















/JvV 














2 


U^| 




1 


!r.V,.\.V.V 










4 ">^~.^,-~--- 












_ -a i a 



FIG. 6. "PUSH-THROUGH" OR DOUBLE SLIDE-CARRIER. 

/ The frame which remains in one position in front of the condenser and 
serves as a container and guide for the "push-through" part. 

2 The movable slide-holder or "push-through" part of the carrier. It 
moves easily to the right and to the left. It contains two slides in the proper 
position for an erect image on a vertical opaque screen (see fig. 7,8}. 

3 3 Notches in the movable part to enable one to grasp the slide easily. 

4 Elevator serving to lift the slide when at either end. In some forms the 
elevator lifts the entire slide from the middle instead of tilting one end. 

5 6 Inclined planes at each end. These raise the elevator when the carrier 
is moved to either end of the base. 

S i Lantern slide in the carrier in front of the condenser. 

S 2 Lantern slide in the carrier at the left end. It is of the first magic 
lantern (1665) and is in position to be removed or to be pushed to the right for 
exhibition. 

30. To start the arc light. Turn on the current by closing 
the switch (fig. 1-4). If the lam]) is of the automatic type the 



24 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I 

magnetic release will allow the carbons to come in contact and 
separate slightly so that the arc will be of the correct length. 

If the lamp is of the hand-feed type the operator must start it 
by bringing the carbons in contact and then separating them a 
short distance (3 to 4 mm.; }/% in.). This is done by turning the 
feed screws by hand (fig. 3, F. S.). 

31. Managing the arc lamp during the exhibition. For an 

automatic lamp, the operator has only to close the switch to start, 
and to open the switch to stop the lamp. The automatic mechan- 
ism is supposed to keep the lamp burning in the best manner. 
From the uneven burning of the carbons it is sometimes necessary 
to make slight adjustments by hand even with automatic lamps. 
This is easily accomplished by turning the proper screws present 
for the purpose (fig. 3, F. S., L. A., V. A.). 

For the hand-feed lamp the operator must bring the carbons 
closer together every four to five minutes or oftener by turning the 
feed screws. If this is not done the distance between the carbons 
soon becomes too great for the current to pass, and the lamp will go 
out. Allowing the lamp to go out when it should not is one of 
the things for the operator to avoid. 

32. Amount of current to use. This depends upon the kind 
of arc lamp used (Ch. XIII), the screen distance, and the character 
of the lantern slides. For dark lantern slides or long distances 
more current must be used than for clear lantern slides and short 
distances. 

For a screen distance up to 10 meters (33 ft.) and a right-angled 
arc lamp (fig. 1-3) one will rarely need more than 12 amperes. 
For a screen distance of 1 5 to 25 meters (50 to 80 ft.), 1 5 or at most 
20 amperes should suffice. If more than 20 amperes are needed to 
give the proper brilliancy to the screen images something is wrong 
with the slides, the room, or the lantern itself, or more probably 
with the management of the lantern. (See under Ammeter 7). 

33. When to light the lamp. The room should never be 
totally dark during an exhibition. The incandescent lamp men- 



CH. I] 



MAGIC LANTERN WITH DIRECT CURRENT 



tioned above ( 16) will avoid this; and furthermore, the operator 
should start the arc lamp before the lecturer turns off the room 
lights ( 25). 

34. When to put the lamp out. The operator should not turn 
out the arc lamp until the lecturer turns on the room lights. The 
intervals of total darkness so common in exhibitions can be avoided 
by keeping in mind the suggestions in this and the previous section. 

It is also a good plan for the operator to remove the last slide 
when the lecturer is through with it, and show a blank disc of light. 
This will inform the lecturer that all the slides have been exhibited 
and give him the hint to turn on the room lights. 



To determine how a lantern slide 
should be placed in the carrier to give 
an erect image on the screen : 

Look through the lantern slide toward 
something light. Turn it until the 
picture is right side up and the print 
reads right, as in this model. 

Then turn the slide so that the bottom 
edge is uppermost like the next model. 



FIG. 7. STANDARD AMERICAN LANTERN SLIDE, FULL SIZE, WITH 
DIRECTIONS FOR INSERTING IT IN THE CARRIER so THAT 
THE SCREEN IMAGE WILL BE ERECT. 



26 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I 




FIG. 8. STANDARD AMERICAN LANTERN SLIDE, FULL SIZE, WITH 

DIRECTIONS FOR MARKING IT, AND INSERTING IT IN THE 

CARRIER so THAT THE SCREEN IMAGE WILL BE ERECT. 

35. Correct position of the lantern slide in the carrier. In 

order that the image on the screen may be right side up and like the 
original in every way, the lantern slide must be put into the carrier 
in the following manner to counterbalance the inverting effect of 
the projection objective (fig. i). 

1 . A lantern slide with any printing upon it must have the side 

which reads correctly face the lamp, if the screen is of 

ordinary form. 

If the screen is translucent like ground glass and the picture is 
viewed from the back of the screen, then the printing must face the 
screen, not the lamp. 

2. In all cases the slide must be put into the holder with the 

bottom edge up (fig. 6, 8). 



CH. I] MAGIC LANTERN WITH DIRECT CURRENT 
1O CENTIMETER RULE 



27 




The upper edge ts m millimeters, the lower in centimeters. 

FIG. 9. SCREEN IMAGE OF A LANTERN SLIDE CORRECTLY INSERTED 
IN THE CARRIER (FiG. 6-8). 

ni j9/v\O[ aq; 'sj3j9tni[[im ui si aSpa aaddn 




FIG. 10. LANTERN SLIDE IMAGE, WRONG EDGE UP. 
3.IUH H3T31/:iTtt3O OI 




ni iswol ?d) .aisJarnillim ni zi sg 
FIG. ii. LANTERN SLIDE IMAGE, FACING IN THE WRONG DIRECTION. 
JJJG nbbei. eqSc 12 lu mi]]inj&(Gi.3' rpG JOMGL in CGnfiraGfGi.3' 




FIG. 12. LANTERN SLIDE IMAGE, WRONG EDGE UP AND FACING IN 
THE WRONG DIRECTION. 

FIGURES 9-10-11-12. LANTERN SLIDES OF A METRIC RULE FULL SIZE. 

The figures show the image as it appears on an opaque vertical screen in each 
of the four possible ways of inserting the slide in the carrier. 

For a translucent screen, or when a mirror is used, the slide in fig. 11 would 
appear erect. 



28 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I 

3. In the slide-changer of the Spencer Lens Co.'s magic lan- 
terns (Delineascopes) , the slide is laid flat, with the face 
up, i. e., so it will be toward the condenser when ready for 
projection. The edge which is to be uppermost in the 
ordinary vertical carrier, is toward the screen. Now when 
this slide-changer is used it turns the slide up in the ver- 
tical position so that it is in precisely the same position as 
with the ordinary slide carrier. 

36. Possible ways of inserting American lantern slides in the 
slide - carrier. The standard American lantern slide is oblong 
(10x8.2 cm.; 4x3^ in.), and the carriers are constructed to 
receive them lengthwise. While they would never be inserted 
with the short edge up, they can be inserted with either long edge 
up, and facing in either direction. This gives four possible posi- 
tions in the carrier, only one of which is correct. That is, there are 
three wrong ways of inserting the slide in the carrier with the 
corresponding wrong images on the screen. It is not very uncom- 
mon for an audience to see all possible images of the same slide, and 
occasionally the wrong ones repeated once or twice. This is as 
inexcusable as it is unnecessary (fig. 10-12). 

37. Possible ways of inserting the square English lantern 
slides. These slides are 8.3 x 8.3 cm. (3^x3^ in.), and being 
square they may be put into the carrier with any of the four edges 
up, and of course with either face toward the lamp. This gives 
eight possible ways of insertion, seven of which are wrong. Square 
slides must have two "spots," (see fig. 13). 

38. Focusing the image on the screen. When the lantern 
slide is in the correct position before the condenser (fig. 1-2) the 
objective must be at such a distance from the slide that the screen 
image will be sharp, and show clearly the printed matter and all 
the details of the picture. With the usual magic lantern the 
objective is nearly in the right position all of the time. But for 
any necessary final focusing there is a rack and pinion on the 
objective, or it is mounted in a tube with spiral movement. By 
turning the milled head of the pinion, or by turning the objective 



CH. I] MAGIC LANTERN WITH DIRECT CURRENT 29 




FIG. 13. SQUARE ENGLISH LANTERN SLIDE FULL SIZE. 

This figure shows the method of "spotting" or marking by the English 
Photographic Club. That is, there are two marks on the upper front margin 
of the slide. Two marks are necessary for square slides, while a single one 
answers for oblong slides. 

The picture on the lantern slide is of a retouching frame to hold the slides 
while being colored. 

in its spiral casing the image may be made perfectly sharp, provided 
that the light is good and the objective also good. With an 
imperfectly corrected objective the margins of the screen image are 
liable to be lacking in sharpness although the middle may be good. 

It may be necessary to focus slightly for each individual slide, 
but ordinarily if one slide is in perfect focus those following will 
also give good images. 

If the screen distance is small (three to five meters ; 10 to 16 feet) 
it may be necessary to focus slightly for each slide if the sharpest 



30 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I 

images are desired. When, however, the screen distance is 10 
meters (30 ft.) or over, it is not usually necessary to focus for each 
slide. 

If the screen distance is very great (20 meters; 65 ft. or more) 
the operator cannot tell by his eye alone when the screen image is 
perfectly sharp. In such a case he must have an assistant stand 
near the screen to tell him when the image is sharp, or he can use 
good opera-glasses and determine for himself. 

When the focus is once found for these long distances it is well 
to mark in some way the exact position of the objective ; then in 
future the operator can be sure of good screen images in the same 
position provided the lantern has not been moved. 

39. Hints on running the lantern for a demonstration lecture. 

It frequently happens that in a demonstration lecture, slides are 
to be shown at several different times. Ordinarily the arc lamp is 
turned out during the intervals ; but to make sure that the desired 
slide can be shown without delay, the arc lamp can be left burning 
all the time, and to avoid lighting the screen a mask can be put in 
front of the objective (fig. 14). A "push-through" carrier (fig. 6) 
should be used, and the next slide to be shown put in one of the 
compartments. The other compartment is left vacant, and this 
empty compartment is put in front of the condenser. If the slide 
were left in position all the time it might become over heated and 
break. 

Whenever the slide is called for it is pushed into position and 
the mask turned aside. This will bring the picture on the screen 
almost instantly. 

A mask or shield for the objective is much more important for 
the slow starting lights like the Nernst, than for the arc ( 146, 
169, 202, 217). 

40. Collecting and arranging the lantern slides at the close 
of an exhibition. After the exhibition is over be sure to remove 
the last lantern slide from the slide-carrier. It not infrequently 
happens that the last slide is left in the carrier, and the lecturer's 
set is thus rendered incomplete. 



CH. I] MAGIC LANTERN WITH DIRECT CURRENT 



3 1 



It should be a part of regular routine to look in the slide carrier 
at the close of'everv exhibition to make sure that the last lantern 

tr- ^ ^ 

slide has-been removed. 




FIG. 14. SHIELD FOR THE OBJECTIVE IN INTER- 
MITTENT PROJECTION WITH SLOW- 
LIGHTING RADIANTS. 

S l Shield raised to allow the light to pass from the objective to the screen. 

S Shield down in front of the objective to cut off the light from the screen. 

The shield should be of a concave form and in front of the objective a short 
distance to avoid heating. It should be made of metal or asbestos and be 
hinged so that it can be easily turned up or down. 



This is also the best time to arrange the slides in the box or a 
pile exactly as they were at the beginning of the exhibition ; then 
the set will be ready for use at the next lecture or demonstration. 

41. Lantern slides permanently fixed in individual carriers. 

Originally lantern slides were mounted in wooden frames. Each 
slide then had its own carrier, which was inserted in a special 
opening for it next the condenser (fig. 15, 32). This method of 
mounting slides still prevails for some purposes. If one wishes to 
use them in the ordinary lantern the common slide-carrier (fig. 6) 
is removed entirely ; then each slide in its carrier is inserted in 
order during the exhibition. This method of mounting is 
admirable for a small collection of slides, as the wooden frame pro- 
tects them, but for a large collection they take too much space and 
arc too expensive. 



MAGIC LANTERN WITH DIRECT CURRENT [Cn. I 




FIG. 15. 



LANTERN SLIDE IN PERMANENT WOODEN 
CARRIER; ONE-HALF SIZE. 



/ Face view of the carrier and its slide. 

2 Sectional view of the carrier, showing the shelf on which the slide rests, 
and the wire spring above. 

The slide is usually cut in circular form, and fitted into a circular opening 
in the frame. A hole of the desired size is first made in the middle of the 
carrier, but not going clear through ; then a slightly smaller hole goes entirely 
through. This leaves a narrow shelf for supporting the slide. Above the 
slide is placed a cover-glass, and then a wire spring to hold the glass in position. 



PROJECTION OF HORIZONTAL OBJECTS 

42. The ordinary magic lantern is in a horizontal position 
(fig. i), but the lantern slide must then be vertical. Objects in 
liquids, and some other objects cannot be put in a vertical position, 
hence the necessity of a rearrangement of the lantern parts so that 
the object may be placed horizontally. This is accomplished by 
placing the second or terminal part of the condenser, in a horizontal 
position, and the projection objective is made vertical. By means 
of a plane mirror in the path of the beam of light from the first part 
of the condenser, the light is reflected vertically upward. The 
object is placed horizontally just above the second element of the 
condenser. The vertical projection objective would give a picture 



CH. I] MAGIC LANTERN WITH DIRECT CURRENT 



33 



on the ceiling above, but by means of another mirror at 45 degrees 
or a prism this vertically directed light is reflected horizontally to 
the ordinary vertical screen (fig. 16, 4aa). (For projection with 
the vertical microscope see 397). 




FIG. 1 6. ARRANGEMENT OF THE MAGIC LANTERN FOR HORIZONTAL 
OBJECTS. 

(Cut loaned by C. H. Stoelting Co.). 

Commencing at the left the parts are : 
L Hand-feed lamp with right-angled carbons. 
H Lamp -house cut away to show the lamp within. 
/ 2 Adjusting screws to move the carbons, 
j 4 Screws for centering the crater. 

5 Adjusting screw for moving the lamp toward and from the condenser. 
C The plano-convex lens of the condenser next the radiant. It here gives 
a parallel beam. 

T Water-cell in the path of the parallel beam. 



42a. In England and America this is often called vertical projection from 
the position of the objective; in Germany it is called horizontal projection 
from the position of the object. 



34 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I 

Af, 45 degree mirror to reflect the parallel beam vertically. 

C, Second element of the condenser in a horizontal position. The lantern 
slide is put just above it. 

O Projection objective in a vertical position for opaque projection. 

M 45 degree mirror above the objective to reflect the light horizontally 
to the screen. 

G Vertical support for the condenser, objective and mirror. 

E Lantern front holding the objective. 

E 2 Set screws for holding the objective in position when once centered. 

M Mirror in horizontal position. When raised 45 it serves to reflect the 
horizontal beam down upon an opaque object. 

C 2 Second element of the condenser used in projection with the microscope 
or lantern objective with the object in the ordinary vertical position. 

S Opening for the lantern slide carrier. 

Z>, Objective and its holder. 

O Projection objective for lantern slides. 

FFF Supports of the condenser, etc. 

N Platform on which opaque objects are placed. 

J5 i; 2 Legs or supports of the prismatic rod serving as an optical bench. 

PROJECTION WITH MULTIPLE LANTERNS 

In the period before the common use of the moving picture 
machine, when the pictorial effect was dependent wholly on the 
magic lantern, two and even more lanterns were run simultaneously 
i. c., both were going all the time. 

43. Composition of multiple lanterns. 

1. Each lantern must be complete in itself. 

2. The size of image of each lantern must be exactly the same. 

3 . The lanterns must be so placed and so inclined toward each 

other that the light discs on the screen exactly coincide. 
They are now usually placed one above the other (fig. 17). 

44. Wiring for multiple lanterns. Each lantern must have 
its own electric lamp. When the supply is no volts or less each 
lamp must be separately wired, and each lamp must also have its 
own rheostat and double-pole knife switch (fig. 2,3). 

In case the supply is 220 volts, each lamp may be separately 
wired as just described; or both lamps may be put in series, i. e., 
along one wire, on one system of wiring, and use but a single 
rheostat. 

45. Use of multiple lanterns. By the use of two lanterns 
there is not shown first one slide and then another simply, but one 



CH. I] " MAGIC LANTERN WITH DIRECT CURRENT 35 

slide seems to melt into the other, hence the name "dissolving 
views." This is brought about by a shutter gradually uncovering 
one objective and at the same time obscuring the other; or, as in 
the figure here shown (fig. 17), by the closing of the iris diaphragm 
of one objective while the other opens. 




FIG. 17. MULTIPLE LANTKKX FOR DISSOLVING 
VIEWS. 

(Cut loaned by the Bausch & Lomb Optical Company). 

Each lantern must have its own arc lamp and rheostat. For dissolving one 
picture into another the iris diaphragm of one objective is opened gradually 
while the other is gradually closed. This is accomplished by pulling up or 
down on the rod connecting the two iris diaphragms in the objectives. 

Some lecture rooms are supplied with double lanterns, not so 
much for the dissolving effect, as for the rapid passage from one 
slide to another. In most cases the "push -through" carrier with a 
single lantern will accomplish this as effectively. 

46. Multiple lanterns for "effects." Formerly certain 
"effects" or striking appearances were produced by the use of two 
or more lanterns which were in operation and projected their light 



36 MAGIC LANTERN WITH DIRECT CURRENT [CH. I 

upon the screen at the same time. For example, to show falling 
snow, in one of the lanterns is a slide showing a landscape, city 
street, etc.. in another is a black band with irregular perforations 
of minute size which give the appearance of snow-flakes. If now 
the light in the lanterns is properly regulated, and the black 
perforated band is moved up over the face of the condenser, 
the snow-flakes will appear to fall either gently or rapidly in the 
landscape or street as one moves the band slowly or rapidly. One 
can give the appearance of a driving storm by tilting the black 
band, for this will make the flakes seem to fall obliquely. 

For rain effects the black band should have slit-like perforations. 

MOVING SLIDES FOR SINGLE LANTERNS 

47. "Effects" with single lanterns. The appearance of 
movement may also be produced in a single lantern. For this two 
slides must be superposed, and one moved over the other. By 
this means various combinations of designs may be made, and also 
appearances of relative movement. Here, naturally, the two 
slides must be close together, or one will be too much out of focus. 
Special slide carriers are constructed for showing these single- 
lantern "effects." 

For simple experiments use a single slide-carrier. The slides 
should have no cover-glass, but may be varnished. Then one 
slide is put in place as for an ordinary exhibition, and another is 
inserted over it and pushed by the fingers into different positions 
to show various combinations. For this experiment the bellows 
between the slide-carrier and the objective should be removed to 
give freedom to the hands in making the various changes necessary. 

48. "Slip-slides" for optical deceptions. Slides with lines 
at various angles, etc., are used to demonstrate these. The lines 
can be shown separately, and then by pushing one slide over the 
other one can get various combinations. For suggestions as to 
slides the reader is referred to works on physiology and experi- 
mental psychology under "optical deceptions." 

49. Most of the "effects" produced by the movement of two 
slides over each other, and the use of multiple lanterns are so far 



CH. I] STEREOSCOPIC SCREEN IMAGES 37 

exceeded in every way by the moving picture that it is hardly worth 
while to go to the trouble to get together the apparatus and slides 
to show these small "effects" when such wonderful ones are shown 
daily in every moving picture theater. 

The moving picture was originally invented to illustrate scientific 
facts; and the indications now are that it is to become a great 
factor in education by its striking portrayal of the processes of 
nature. (See Ch. XI). 

STEREOSCOPIC SCREEN IMAGES 

50. For a stereoscopic screen image the same fundamental 
law must be observed as for any other stereoscopic effect. That 
is, there must be two slightly different images corresponding with 
the image seen by the left eye and that seen by the right eye. 
These images must be projected on the screen so that they nearly 
coincide, then by some means the left eye sees its left-eye image, 
but not the right-eye image; and the right eye sees the right-eye 
image, but not the left-eye image. The two images are then 
combined in the brain and the stereoscopic effect follows as with 
ordinary naked eye binocular vision or when using a stereoscope. 

With the magic lantern this effect has been produced in three 
principal ways: 

(1) By the aid of prism spectacles. Lantern slides of a stereo- 
scopic pair are projected on the screen so that they nearly coincide 
by the use of two lanterns. When this is done some people can 
get the stereoscopic effect by looking at the pictures with the naked 
eye, but for most people it is necessary to look through prism 
spectacles so that the right eye shall see only one image and the 
left eye only one. 

(2) By the aid of polarized light and Nicol-prism spectacles. 
According to this method two lanterns are used and two lantern 
slides, making a stereoscopic pair. For one lantern there is used a 
Nicol-prism or a glass pile and the projection is made with the 
ordinary polarized light. A similar prism or pile is used for the 
other lantern, but the extraordinary polarized light is used for 
projecting its image. These two images are projected so that they 



38 STEREOSCOPIC SCREEN IMAGES [Cn. I 

nearly coincide upon the screen. The screen is covered with silver 
foil to prevent the depolarization of the reflected light. Now to 
look at the screen image and to make it possible for each eye to see 
only its own image, the observer must wear polarizing or analyzing 
spectacles with the prisms or piles corresponding, with the one 
supplying the light for its own image. For example, if the right 
eye image is made by extraordinary polarized light, then the right 
eye of the observer must have its prism spectacle so that it trans- 
mits the extraordinary polarized light, but extinguishes theordinary 
polarized light which produces the left eye image. And the left eye 
must have its prism so that it will receive the polarized light from 
its image, but extinguishes that from the right eye image. Each 
eye then sees its own image, but not the one for the other eye, and 
the conditions for stereoscopic vision are fulfilled. 

(3) The two-color method. For this method two complementary 
colors are selected usually red and green. 

(A) With two lanterns there are projected the two images of a 
stereoscopic pair so that they nearly coincide. There is put some- 
where in the path of the beam of one lantern a plate of red glass and 
in that of the other lantern a plate of green glass. The observer 
must have spectacles or viewing glasses of corresponding colors. 
Then with one eye he sees the red image and with the other the 
green image. The combination of these colored images by the 
brain gives a stereoscopic image in black and white. 

(B) With a single lantern the two-color stereoscopic effect can 
be produced as follows: The two pictures of a stereoscopic pair 
are printed by one of the color processes so that one is a red picture 
and one a green picture. These two are placed together so that 
they nearly coincide, then they are projected by one lantern. 
With the naked eye the pictures look like any two-color picture 
where the colors do not register, and such a screen picture is any- 
thing but satisfactory; but now if spectacles or viewing glasses of 
corresponding colors arc held before the eyes, one eye sees the green 
picture and one eye the red picture and the stereoscopic effect 
comes out very strikingly. 

The simplest way to determine which color to put in front of the 
right and which in front of the left eye is to try first one color then 



CH. I] CENTERING THE MAGIC LANTERN 39 

the other. In general it will be found that if the red parts are at 
the right then the red glass must be over the right eye and similarly 
for the green. Presumably if one used the wrong color then there 
should be a pseudoscopic effect, convex objects looking concave, 
etc.; but this effect is difficult to obtain. 

It is seen that in all these methods the observer must be supplied 
with some means by which only one of the projected images is seen 
by one eye, the other by the other eye. Stereoscopic projection is 
necessarily, therefore, expensive. 

For most people any good lantern slide shows perspective and 
relief sufficiently. 

CENTERING THE VARIOUS PARTS OF THE LANTERN AND 
SEPARATING THEM THE PROPER DISTANCE 

51. Centering. By this is meant the arrangement of the 
source of light, the condenser and the projection objective so that 
the source of light, and the principal optic axis of the condenser and 
of the objective shall be in one straight line, and each lens be 
perpendicular to that straight line (fig. 1-4). 

When the different elements are once centered along one straight 
line the objective and the condenser should be fixed in position so 
that they cannot be raised or lowered or turned sidewise. If the 
source of light gets slightly out of center by the burning of the 
carbons, it may be recentered by bringing the carbons nearer 
together or by regulating the position by the fine adjustments of 
the lamp. 

In the right-angled arc lamp the upper carbon, which furnishes 
the light, is constantly in the optic axis. With oblique carbons 
(fig. 39) the source of light constantly shifts with the burning away 
of the carbons ; and with the direct current lamp the source of light 
gradually rises above the axis. With the alternating current and 
V-arranged carbons one source shifts above and one below the axis, 
or one to the right and one to the left depending upon the arrange- 
ment of the V. In centering the lamp one should start with the 
carbons in contact and take the point of contact to center from. 



40 CENTERING THE MAGIC LANTERN [Cn. I 

Remember that one should never change the position of the 
condenser or of the objective to compensate for the lack of center- 
ing of the source of light. 

52. Mechanical method of centering. This is the method 
most satisfactory for both manufacturer and user in getting the 
various parts properly aligned. 

Generally some form of track (optical bench) is used on which 
the various parts are placed and along which they can slide. The 
straight line or axis to which all parts are to be centered is at a 
selected, definite position above the base-board or table supporting 
the track (fig. 3 , 40) . 

The first thing, then, is to decide upon the distance the axis is to 
be above the base-board or table. 

For all work upon centering, the bellows between the condenser 
and the objective should be removed so that the faces of all parts 
can be seen. 

The position of the common axis may be determined by some 
part of the apparatus, such as the condenser. Or one can decide 
upon some convenient level which will give sufficient room for the 
arc lamp and its carbons, and then adjust all parts to this level. A 
good way to get all at the proper height is to make a measure or 
gauge of wood just the height of the axis. If this is a board which 
just fits between the tracks, and has a peg indicating the middle 
point between the tracks it will help to get the parts perpendicular 
to the axis as well as at the right level. If the wooden gauge is 
carefully made it will enable one to center the parts to within one 
or two mm. (Vi to y 24 inch). Very slight variations from perfect 
mechanical centering can be compensated for by using the fine 
adjustment screws of the arc lamp. 

53. Getting the center of the lens faces. This can be done 
by using a rule in millimeters or Meth's inch. Or it can be done by 
pressing some white paper against the lens face and creasing it 
around the edges with the finger. The center of this circle of paper 
can then be found as shown in fig. 18. If the center is marked and 
the paper then put over the lens face one will have a guide to center 
by. 



CH. I] 



CENTERING THE MAGIC LANTERN 




FIG. 1 8. FIGURE SHOWING HOW TO 
FIND THE CENTER OF A CIRCLE. 

Draw two chords (ch ch) and erect perpendiculars at their middle points. 
Where these perpendiculars cross is the center of the circle (C). 

As stated above, when once centered, the objective and con- 
denser should be fixed in position. 

54. Avoidance of obliquity. Not only must all the parts be 
at the same level and in one straight line, but the lenses must be 
perpendicular to that straight line and not oblique. Then the 
straight line or common axis passing from the crater of the upper 
carbon to the screen will coincide with the principal axis of the 
condenser and the projection objective, and the arrangement for 
perfect projection will be realized (fig. 1-4, 26). 

One can usually tell when the parts are in line and not oblique by 
sighting along them with the eye, or by the use of a straight edge 
like a T-square. To make sure by measurement one can put the 
optical bench or base-board (fig. 158, 1 59) , on a level table and next 
a smooth wall. Then by measuring horizontally the central points 
can be determined exactly as their height was determined ( 52). 

CORRECT DISTANCE APART OF THE DIFFERENT ELEMENTS 

55. Radiant and condenser. With the three-lens condenser 
the radiant is at the right distance when it is at the principal focus 
of the first element of the condenser (fig. 2). This will give a 



CENTERING THE MAGIC LANTERN 



[CH. I 




FIG. 19. CONCENTRIC CIRCLES ON THE 

FACE OF THE CONDENSER, SHOWING 

THE SIZE OF THE CIRCLE OF LIGHT 

WITH VARIOUS POSITIONS OF 

THE RADIANT. 

When the radiant is at the proper distance, the entire face of the condenser 
is illuminated (/). 

As the radiant and condenser are separated the part illuminated becomes 
smaller and smaller (2-4). (See also fig. 20). 

cylinder of approximately parallel rays between the two elements 
of the condenser, and will fully light the face of the second element. 
One can determine this easily by putting a sheet of white paper 
over the face of the condenser which is toward the objective. If 
the radiant is in the right place the entire face will be light. If the 
radiant is too far off, only a part of the face will be illuminated 
(fig. 19). If the radiant is too close the face will be lighted, 
but the light will be diverging between the condenser lenses. 
In this case a part of the light falls outside the second element and 
is lost. There is liable also to be a defective screen image (fig. 28). 
One can get the condenser at the right distance from the lamp by 
first separating the lamp and condenser a considerable distance and 
then gradually bringing them closer and closer together until the 
condenser face is just filled with light. Sometimes the radiant is 
put nearer than the principal focal distance on purpose, so as to 
correct in part for the lack of proper proportion between the con- 
denser and the objective ( 56). 



CH. I] CENTERING THE MAGIC LANTERN 43 

With the two-lens condenser used for lantern slides the lamp is 
usually closer than the principal focal distance of the first lens, this 
makes the beam between the lenses diverging, hence it is best to 
have the two lenses as close together as possible to avoid loss of 
light (fig. i). 

With this condenser and diverging light between the lenses the 
only rule that can be given is to adjust the distance between the 
lamp and the condenser until the best light is obtained on the 
screen. If this brings the crater of the arc lamp within 8 to 10 cm. 
(4 in.) of the first lens, then it will be necessary to substitute longer 
focus lenses for either the first or the second condenser lens or for 
both. In general, the first lens should be of about 15 cm. (6 in.) 
focus and the second lens should have a somewhat shorter focal 
length than the projection objective. For example, if the projec- 
tion objective is of 38 cm. (15 in.) focus, the second lens of the con- 
denser in the two-lens form should be of about 2 5-30 cm. (10-1 2 in.) 
focus. This will bring the diverging cone to a focus near the center 
of the objective. 

56. Condenser and projection objective. If the projection 
objective and the condenser are properly proportioned the conden- 
ser will focus the light near the center of the projection objective 
when the lantern slide is in focus on the screen (fig. 1,2). 

If the condenser is of so short a focus that the light from the 
condenser comes to a focus before reaching the objective the field is 
restricted and bordered by a red margin (fig. 29). 

If, on the other hand, the condenser is of too long a focus for the 
objective the light will not come to a focus by the time it reaches 
the center of the objective (fig. 28). In this case the field will be 
restricted and bordered by blue. 

OPTICAL TEST FOR CENTERING 

57. Optical test for centering the radiant and the condenser. 

If these are properly centered along one line, and the two are 
separated a considerable distance when the lamp is burning, the 
light spot on the face of the condenser looking toward the objective 



CENTERING THE MAGIC LANTERN 



[CH. I 




FIG. 20 A. CONDENSER FACE WITH THE SPOT OF LIGHT IN THE MIDDLE, 
SHOWING THAT THE LAMP AND CONDENSER ARE ON THE SAME Axis. 

FIG. 20 B. CONDENSER FACE WITH THE SPOT OF LIGHT OUT OF THE 

CENTER. THIS SHOWS THAT THE CONDENSER AND LAMP ARE NOT ON 

ONE Axis. 

To get the appearance here shown the lamp must be pulled back considerably 
beyond the principal focus of the first element of the condenser. 

will appear in the center (fig. 20) . This can be easily seen by hold- 
ing a piece of paper against the condenser face. In case the two 
are not properly aligned, the white spot on the paper will appear 
outside the center, at the right or left, above or below. On account 
of the inverting effect of lenses the arc light will be too far from the 
center in just the opposite direction from the spot of light. For 
example, in figure 2oB the light spot is too far to the left, 
consequently the crater of the positive carbon must be too far to 
the right. One should change it to the left by the adjusting screws 
until the circle of light appears exactly in the middle (fig. 20 A). 

58. Optical test for centering the condenser and the objec- 
tive. After the condenser and the radiant are properly centered, 
and the radiant put at the principal focus of the condenser one can 
tell whether the objective is on the same axis by looking at both 
ends of the objective when it is at the proper distance from the 
condenser (fig. 1-2). 

If the objective is in line with the lam]) and the condenser the 
spot of light from the condenser can be seen in the middle of the 



CH. I] CENTERING THE MAGIC LANTERN 45 

first lens of the objective. The light should strike the middle of 
the first lens and leave through the middle of the last lens of the 
objective (fig. i). 

If it is not centered the cone of light will strike at one side of the 
center and leave at one side. If it is greatly out of center the cone 
of light may fall wholly outside the objective; this frequently 
occurs in micro-projection. 

To center the objective it should be moved up or down, to the 
right or to the left, until the cone of light strikes it exactly in the 
center and leaves the center. No change of the lamp or the con- 
denser should be made, for that would spoil the centering of those 
two elements. After the objective is centered, it should be fixed 
firmly in position. Any slight variation from the center by the 
irregular burning of the carbons, can be corrected by the fine 
adjusting screws of the lamp (fig. 3, L. A.; V. A.). 

CENTERING THE OBJECTIVE IN A VERTICAL POSITION 

59. When the objective must be made vertical in projecting 
horizontal objects, the radiant and the condenser should first be 
centered as described above ( 55). Then the second element of 
the condenser should be removed and placed in a horizontal posi- 
tion with the convex face downward, and the flat face upward 
toward the objective. A plane mirror at 45 degrees is placed in 
the path of the beam of light from the first element of the con- 
denser. The light will be directed vertically upward. The hori- 
zontal condenser lens must be moved until it receives this vertical 
cylinder of light and continues the central or axial ray in a vertical 
direction. One can tell when this is the condition by pulling the 
arc lamp back from the condenser until a small circle of light 
appears on the horizontal condenser lens (fig. 2oA, B.). If it is 
centered the spot of light will be in the middle. If it is not in the 
middle move the upper lens until it is, but do not change the posi- 
tion of the lamp. When the horizontal lens is centered, move the 
arc lamp up toward the condenser until the horizontal lens is filled 
with light ( 55). 



TROUBLES WITH THE MAGIC LANTERN 



[CH. I 



60. Centering the vertical objective. After the horizontally 
placed condenser lens is centered the objective is placed in a vertical 
position over it and moved sidewise until the cone of light enters 
the middle of the first face and leaves the middle of the last face of 
the objective. One proceeds exactly as for centering it in the 
horizontal position ( 55, 58). Just over the objective is placed a 
45 degree mirror silvered on the face, or a right-angled prism, to 
direct the vertical rays horizontally to the screen (fig. 16). The 
lower mirror may be an ordinary glass mirror silvered on the back, 
but the mirror over the objective must be silvered on the face to 
avoid a duplication of the image. 



TROUBLES: HOW TO AVOID AND HOW TO 
OVERCOME THEM 

THE LAMP CANNOT BE STARTED 

61. This may be because there is no voltage in the main line. 
The presence of current is easily determined by using the testing 
incandescent lamp (fig. 21). An incandescent lamp in the circuit 
as shown in fig. 2 or 4 will show whether the current extends to the 
lamp switch. 





FIG. 21. TESTING INCANDESCENT LAMP. 

W l W? The two supply wires for the lamp. 

For this testing lamp a socket without key switch is best. It is also wise to 
have the lamp protected by a wire guard. The wires at W-, W 2 should be 
exposed only a short distance as shown. 

To test with the lamp put the naked ends of the wires W t W., upon metallic 
parts of the circuit to lie tested being sure to make contact with both conduc- 
tors of the circuit. For example, the two wires or the two blades of a knife 
switch, etc. If there is voltage in the line at that point the lamp will light up. 

62. The connections in the arc lamp may not be good, that is, 
the set screws holding the connecting wires may have become 



CH. I] TROUBLES WITH THE MAGIC LANTERN 47 

loosened, or a wire may have become wholly separated from its 
connections. 

63. A fuse may have burned out somewhere along the line. 
Commencing with the fuse nearest the lamp, take each fuse out and 
examine it. Use the testing incandescent lamp also. 

64. A fuse plug may not be screwed in tightly enough to make 
good contact. Occasionally some one puts a piece of paper or wood 
in the fuse socket, thus preventing metallic contact. Such 
obstructions should be looked for and removed; then the fuse plug 
can be made to produce metallic contact. 

65. The switches may not be properly closed, and hence the 
circuit is not complete. 

66. The carbons may be so short that they cannot be brought 
in contact, and thus the circuit cannot be completed. Put in new 
ones. 

67. The range of the lamp movement may be at its limit, so 
that the carbons cannot be approximated. This must be corrected 
by turning the screws back and then setting the carbons by hand, 
if long enough, or by putting in new carbons. 

68. If one uses an automatic arc lamp, it may be that the 
mechanism does not work. Before looking elsewhere for the 
trouble, one should try the hand-feed device present in all auto- 
matic lamps and make sure that the carbons are brought in con- 
tact and then slightly separated to establish the arc. 

69. Of course, if one uses a hand-feed lamp it will not start 
until one brings the carbons in contact by the proper device for the 
purpose. As soon as the carbons touch there will be a flash of 
light; then the carbons should be slightly separated. 

70. There may be a short circuit in the lamp itself due to a 
burning out of the insulation. This may be detected by opening 
the double-pole knife switch slowly. If there is a big spark when 
the switch finally opens, a short circuit in the lamp is strongly 
indicated. 



48 TROUBLES WITH THE MAGIC LANTERN lCn. I 

Unless one has considerable knowledge of arc lamps it is advis- 
able to get an electrician to repair the lamp. 

Short circuiting in the lamp is a rare trouble and less liable to 
occur than almost anything else. 

GOING OUT OF THE LAMP 

71. This may be due to the stopping of the dynamo. 

72. A fuse may burn out somewhere along the line. 

73. Some connection may burn out or one or both wires may 
be disconnected. 

74. The carbons may have burned off so that the interval 
between the ends is too great for the current to pass. This is a 
very common cause, and is, of course, easily remedied by the use 
of the feeding screws of the lamp to bring them closer together. 
If the carbons are so short that they cannot be brought together, 
new carbons must be inserted. Always open the table switch 
before putting in new carbons. 

Sometimes the screw holding the lower carbon is not set up 
enough and the carbon falls down. If this is the trouble open the 
table switch and replace the lower carbon in its proper position and 
tighten well the set screw holding it. 

Always look at the carbons first in case the lamp goes out 
unexpectedly (see also above 66-67, 7 an d an the causes for no 
current 61-70). 

NOT ENOUGH CURRENT 

75. There may not be enough in the line. 

76. The line may be grounded. Test for this with the testing 
incandescent by touching one of the terminal wires of the incan- 
descent to some metal object connected with the ground, like the 
metal tube enclosing the wires, a water or gas pipe or radiator, and 
the other to one of the exposed metal parts of the conductors, first 
on one side and then on the other. If there is a connection of 
either wire with the ground the testing lamp will light when its two 
wires are connected, one with the radiator, etc., and the other with 



CH. I] 



TROUBLES WITH THE MAGIC LANTERN 



49 



the line wire which is not grounded. In some cases one wire is 
purposely grounded. In such cases great care must be taken not 
to ground the other wire (see also fig, 266-267 689). 

77. There may be too much resistance in the circuit. Open 
the rheostat wider, if it is adjustable (fig. 281), keeping an eye on 
the ammeter to see when the current is of the desired amperage. 




FIG. 22. INCLINED AND VERTICAL CARBONS 
IN THE CORRECT RELATIVE 
POSITION. 

The upper carbon is positive and supplies the light in both cases. 




FIG. 23. CARBONS IN THE CORRECT RELATIVE POSITION FOR BOTH 
DIRECT AND ALTERNATING CURRENTS. 

A Inclined carbons in the correct position for alternating current. 

B Inclined carbons in the correct position for direct current. 

C Carbons at right angles in the correct position for either direct or 
alternating current. Direct current is indicated. 

D Carbons arranged in a V-shaped position. For this position alternating 
current only is employed; and the crater on each carbon contributes to the 
light. The V may be either in a vertical or in a horizontal plane. The ver- 
tical arrangement is the more common. 



TROUBLES WITH THE MAGIC LANTERN lCn. I 




FIG. 24. CARBONS IN BAD POSITION; THE UPPER CARBON CUTTING 

OFF THE LIGHT FROM THE UPPER PART OF THE CONDENSER, AND 

HENCE CASTING A SHADOW ON THE LOWER PART OF THE SCREEN. 

A Carbons at an inclination of about 25 degrees, with the upper or positive 
carbon too far forward. 

B Carbons at right angles, with the upper carbon too far forward. 

S Screen image of the condenser face. As the upper carbon is in the way, 
the upper part of the condenser is partly in shadow, and hence the screen image 
will be shaded on its lower part due to the inverting action of the objective. 




FIG. 25. CARBONS IN BAD RELATIVE POSITION, THE LOWER OR NEGATIVE 
CARBON EXTENDING UP IN FRONT OF THE POSITIVE CARBON. 

A Carbons at right angles, with the lower carbon too high. 

B Both carbons vertical, but the lower or negative one standing in front 
of the upper one. 

.V Screen image of the condenser face. As the condenser is not well lighted 
on its lower part due to the shading action by the lower carbon, the screen image 
will be shaded correspondingly on its tipper part due to the inverting action of 
the objective. 



CH. I] TROUBLES WITH THE MAGIC LANTERN 51 

IRREGULAR OR INSUFFICIENT LIGHT ON THE SCREEN 

78. There may be an insufficient current flowing through the 
lamp. Consult the ammeter ( 7, 75-77). 

79. Improper relative position of the carbons. Look at them 
occasionally through the window in the lamp house. They should 
be in the relative position shown in fig. 23. If they are in a wrong 
position (fig. 24, 25), one cannot expect to get a good screen light. 

It sometimes happens that one or both of the carbons has no soft 
core, although the hole in the carbon is present. In such a case the 
crater is liable to jump around as with a solid carbon. Easily 
corrected by substituting a properly cored carbon. 

80. Wrong polarity of the supply wires. As stated above 
(5) the positive supply wire should be connected with the lamp 
so that the current passes along the upper carbon and from its tip 
over to the lower carbon, whence by means of the negative wire, it 
passes back to the generator or dynamo. In case the wires were 
reversed in position, the lower carbon would be positive and the 
bright crater would be on it. This would give a poor light, for the 
crater would not face the condenser, and as this carbon would burn 
away more rapidly than the upper carbon the upper one would soon 
be in the position shown in fig. 246. There would then be a 
double reason for a poor screen image, viz., the crater would not 
face the condenser, and the upper carbon would act as a shield to 
cut the light off the condenser. To determine whether the wires 
are connected to the lamp properly, insert carbons, turn on the 
current, and let the lamp burn a minute or two. Then turn off 
the lamp and watch the hot ends of the carbons. The positive one 
will remain red hot longest. (See also Ch. XIII, 701-703 for 
determining the polarity). In case the lower carbon remains 
glowing longer than the upper, the polarity is wrong (fig. 271). 

Open the switch and remove both wires from their binding posts 
and insert them in the reverse position. Then repeat the experi- 
ment and the upper carbon should remain glowing longest. 

After one has had some experience it is easy to tell whether or not 
the wires are properly connected by watching the carbons through 



$2 TROUBLEvS WITH THE MAGIC LANTERN [Cn. I 

the lamp-house window when the lamp is burning. The upper 
carbon should always be considerably brighter than the lower one. 
When one has found the correct polarity it is wise to mark the 
positive wire red and the negative wire black. It is also a good 
plan to mark the positive switch connections plus with red and the 
negative connections minus with black. But one must not forget 
that the polarity is liable to be changed by the changing of the 
wires in the main line when repairs are made, so one must be on the 
alert to detect polarity change. 

81. Non-registering of the direct current ammeter. In first 
installing an ammeter if the hand does not register on the dial when 
the current is turned on and the arc lamp started, either the 
instrument is out of order, or more likely the wires are wrongly con- 
nected. Remember that the ammeter must be inserted in one 
wire, then if it does not register when the lamp is burning the wires 
were inserted wrong. Turn off the current and reverse the wires 
in the binding posts of the ammeter. If now the wires are properly 
connected both to the ammeter and the arc lamp, the polarity in 
both will be changed by a change in polarity in the main line, and 
the wires must be changed around in the binding posts in the 
ammeter and in the arc lamp to get the polarity correct in both. 
As the lamp and the ammeter are wholly independent instruments, 
the polarity may be correct in both or wrong in both, or correct in 
one and wrong in the other. (See also Ch. XIII, ;o2a for ammeter 
which can be used with both alternating and direct current) . 

DEFECTIVE OPTICAL RESULTS 

82. There may be direct light falling on the screen from some 
window or some lighted lamp in the room . This will make the disc 
of light, or the lantern picture on that part of the screen receiving 
the adventitious light, look faded or gray instead of brilliant. It 
will look as if that part of the screen were not so brilliantly illumin- 
ated, when, in fact, more light may be falling on it. To be effec- 
tive the light must reach the screen from the lantern and from no 
other source. 



CH. I] 



TROUBLES WITH THE MAGIC LANTERN 



53 



SHADOWS AND RESTRICTION IN THE Disc OF LIGHT ON THE SCREEN 
83. The radiant, i. e., the crater of the upper carbon (fig. 27) 
may be outside the main axis (above, below, to the right or to the 
left of it). If sufficiently outside the center there will be only an 
elliptical light area present. On the side toward which the crater 
is displaced there will be a blue crescent or spot, and on the oppo- 
site side a dark crescent, bordered, in extreme cases, by red. 
Remedy: get the crater back in the axis. 

84. The condenser may be out of center. This will give the 
same defective light on the screen as when the light source is off 





FIG. 26 (A). DIAGRAM OF A MAGIC LANTERN AND A SCREEN IMAGE WHEN 
ALL THE PARTS ARE IN CORRECT PROPORTION AND ON ONE Axis. 

Axis The common axis passing from the radiant along the principal axis 
of the condenser and the objective to the screen. 

C Condenser of three lenses, the first clement (L,) composed of a meniscus 
and a plano-convex; the second element (LJ, is a plano-convex. The con- 
vex surfaces face each other as usual. 

F Principal focal distance of the condenser. 

Projection objective. 

R Radiant giving the light. 

S The screen fully and perfectly lighted. 

FIG. 27 (B). DIAGRAM SHOWING THE EFFECT OF HAVING THE RADIANT 
BELOW THE Axis. 

There appears a blue shadow on the lower part of the screen (S). 

Whenever the radiant is off the axis the dark blue shadow will be on the 
corresponding side of the screen. In this case the radiant would have to be 
raised to get rid of the shadow. If the shadow were on the left it would be 
necessary to move the radiant to the right and so on. 



54 



TROUBLES WITH THE MAGIC LANTERN lCn 1 





FIG. 28 (A). DIAGRAM SHOWING THE EFFECT ON THE SCREEN IMAGE 
WHEN THE RADIANT is TOO NEAR THE CONDENSER. 

In this case the conjugate focus of the condenser (/) is considerably farther 
off, i. e., beyond the objective, the screen image is made smaller, and the light 
disc on the screen is bordered with blue. With some condensers there is a 
dark or blue disc in the center also. (Lettering as in fig. 26). 

FIG. 29 (B). DIAGRAM SHOWING THE EFFECT ON THE SCREEN IMAGE 

WHEN THE RADIANT is BEYOND THE PRINCIPAL Focus 

OF THE CONDENSER. 

This brings the conjugate focus of the condenser (/) nearer the condenser, 
and in this case just before the light reaches the objective. It narrows the 
screen image and the light disc is bordered with red. (Lettering as in fig. 26). 

the axis, but the blue spot or disc will be on the side away from 
which the condenser is displaced, being just the reverse of the 
position when the light source is off the axis ( 83). 

If the condenser is too high the blue spot or disc will be on the 
lower part of the screen; and if the condenser is too low the blue 
edge will appear on the upper part of the screen ; if to the right the 
blue disc will be at the left, etc. That is the condenser inverts the 
position (fig. 27). 

The condenser should be correctly centered once for all and 
firmly fixed in position so that it need never be changed. 

85. The projection objective may be off the main axis. The 

effect will be the same as when the source of light is off the axis. 
This is due to the fact that while the condenser inverts the rays, 



CH. I] TROUBLES WITH THE MAGIC LANTERN 55 

they are re-inverted or erected by the objective. If the condenser 
and source of light are on one axis and the objective off that axis, 
it must be recentered; but as stated above ( 54) when the objec- 
tive and condenser are once centered they should be fixed in posi- 
tion, then the only element of the lantern to become decentered is 
the crater of the arc lamp, i. e., the source of light. The fine 
adjustment screws of the lamp will enable one to center the light. 
By limiting the changes to one element, viz., the source of light, 
corrections can be made quickly and accurately. If one tries to 
get the light centered by changing two or all three of the elements it 
leads only to chaos. 

86. For the effects of spherical aberration and for a ghost, a 
white or black spot in the center of the field, see also 828. 

87. The radiant (i. e., crater of the upper carbon) may be too 
close to the condenser. This will give a restricted field with a blue 
margin or there may be a blue circle in the center of the disc (fig. 
29, 30). 

88. The radiant may be too far from the condenser. This 
will produce a restricted screen disc with the edge bordered with 
red (fig. 29). It is easily corrected by bringing the radiant and 
condenser closer together. 

89. The condenser may be of too short focus, so that the 
light comes to a focus before reaching the objective when the 
lantern slide is in focus (see 56, fig. 29, 30). Correct the defect 
by using a lens of longer focus for the second element of the con- 
denser. It may be less satisfactorily compensated for by putting 
the radiant nearer the condenser. 

90. The condenser may be of too long a focus (see 56, 
fig. 28). Correct by using a shorter focus condenser. It may also 
be compensated for in part by removing the radiant farther from 
the condenser, but this lessens the available light. 

91. There may be dirt, mist or opacities on some of the glass 
surfaces. This is easily remedied by cleaning the glass. 



TROUBLES WITH THE MAGIC LANTERN 



[On. I 




FIG. 30. ARRANGEMENT AND CENTERING OF THE RADIANT. 

(From the catalogue of Fuess). 

(/) The Radiant, i. e., the crater is too far to the right. 
(2) The crater is too far to the left, 
(j) The crater is too high. 

(4) The crater is too low. 

(5) The crater is too far from the lamp condenser. 
(6-7) The crater is too near the condenser. 

(8) The crater is in the correct position. 

One of the condenser lenses may be cracked. If a new lens can- 
not be inserted, but the cracked one must be used at the time, 
rotate it around until the crack is least noticeable. 

There may be strings or wires hanging down in the path of the 
beam of light. They will give sharp shadows on the screen. 
Remove them. 

92. Defective or too opaque lantern slide. The lantern 
slides may be cracked, producing a dark streak through the picture. 
There may be dirt or mist on one or more of the glass surfaces. 

The slide may be too opaque. There is a tendency to make 
lantern slides so opaque that only the most powerful radiants can 
give anything like satisfactory screen images. This is a great 
mistake. Lantern slides properly made arc very transparent and 
show all the delicate shading, from the densest to pure transparency 
(clear glass). Probably 99 slides are too dense where one is not 
dense enough. The opacity of the slides made by the autochrome 



CH. I] TROUBLES WITH THE MAGIC LANTERN 57 

or starch process is one of their great drawbacks. Only powerful 
radiants give satisfactory screen images. 

93. Shadow on the screen with water-cell. In case the water 
in the water cell has evaporated in part there will be a very dis- 
agreeable shadow on the lower part of the screen (fig. 31). It is on 
the lower part of the screen although it is the upper part of the water 
cell that will be empty. This is due to the inverting action of the 
objective. 




FIG. 31. SHADOW ON THE LOWER PART OF THE SCREEN WHEN THE 
WATER-CELL is BUT PARTLY FILLED. 

S Screen image with shadow on the lower side. The water is of course 
present in the lower part of the water cell, and absent from the upper part; 
but, owing to the inversion produced by the objective, the screen image shows 
the shadow on the lower part. 

Occasionally the water is entirely absent from the water-cell. 
Then there will be a very poor screen image, the entire screen being 
affected by the obscurities on the dry surfaces of the water-cell. 

BREAKING OF CONDENSER LENSES 

94. It is usually the lens next the radiant that cracks or 
becomes shattered. This is due to the too rapid heating or cooling 
of the condenser lens, or to the mounting, which may be too rigid 
to allow of free expansion of the lens as it becomes hot. 

Condenser lenses are especially liable to break: (i) When too 
heavy currents are used; (2) when the lamp-house is not well and 
evenly ventilated; (3) when currents of cold air strike the hot 
condenser; (4) when the lens mounting is not provided with 
ventilating openings for free circulation of air between the lenses; 



58 TROUBLES WITH THE MAGIC LANTERN [Cn. I 

(5) when the lens next the radiant is of such a focus that the 
lamp must be put very close to it. 

95. Unequal heating. Breakage often occurs from unequal 
heating of the lens. This is perhaps as common with large flame 
sources such as the kerosene flame, the al co-radiant or Welsbach 
mantle gas flame as with the electric arc. With the electric arc, 
if the crater is too close to the lens the thick central part of the lens 
expands rapidly before the edge is heated enough to expand with 
the middle part. Separating the lamp and condenser somewhat, 
for a few minutes after starting the lamp would give the condenser 
a chance to expand uniformly. 

96. Mounting of the lenses. This may not give the lenses 
sufficient freedom of expansion. In all forms of condensers as now 
constructed there is almost invariably provision for this expansion, 
and for free circulation of air between the lenses. The lens next 
the radiant is usually held by a few obliquely extending springs, 
(fig. 36 B), thus giving the greatest freedom. To prevent break- 
age some operators avoid all direct contact of the condenser with 
the metal mounting by the use of asbestos paper. Others think 
that a heavy metal ring around the edge of the condenser will 
lessen breakage by preventing the too rapid cooling. 

The final solution of condenser breakage will come when the 
glass makers produce heat-resisting, optical glass. 

97. Breakage due to reversing the ends of the condenser. 

That is, the condenser lens which should be next the projection 
objective is put next the lamp. The lens which should be next the 
lamp is specially mounted for expansion ( 96). Furthermore, the 
condenser is not designed optically in most cases so that it will give 
equally good results if reversed. In the magic lantern the lens next 
the objective has frequently a longer focus than the one next the 
radiant, so that a reversal injures the optical effect as well as 
endangers the condenser. 

If the makers of projection apparatus would so construct their 
condenser mountings that they could not be reversed, the)- would 
be doing a friendly service to many. 



CH. I] 



SOME AMERICAN MAGIC LANTERNS 



59 



98. If the lantern table is on a concrete floor which is damp 
the operator is liable to get a shock unless he stands on a mat or 
board or other insulating material, provided some part of the cir- 
cuit is grounded (see 689). 



SOME EXAMPLES OF AMERICAN MAGIC LANTERNS FOR THE DIRECT 
CURRENT ARC LAMP 

99. The following examples of American Magic Lanterns are 
introduced to give the reader some notion of the lanterns on the 
market which can be obtained at any time and at a very moderate 
cost. 

In subsequent chapters will be found pictures of lanterns for 
the different forms of radiants, and for two or more kinds of 
projection (combination apparatus). 

In the appendix at the end of the book will be found the addresses 
of some of the great manufacturers in all countries with the prices 
for the different complete outfits for the various forms of projection. 




FIG. 32. MAGIC LANTERN IN OUTLINE TO SHOW THE PARTS. 
(Cut loaned by Williams, Brown & Earle). 

At the left, the side of the lamp-house is removed to show the hand-feed, 
right-angled arc lamp with the supply-wires and the carbons in position. 

C D The condenser composed of two plano-convex lenses. In the space 
(0) a water-cell may be inserted. 

G The oblong opening, just in front of the condenser, into which the slide 
carrier is inserted. 

A The projection objective fastened to the end piece B, which also holds 
the bellows. 

E F Set screws serving to fix the apparatus on the guide rods. 



6o 



SOME AMERICAN MAGIC LANTERNS 



[CH. I 




FIG. 33. SIMPLE MAGIC LANTERN WITH A TWO-LENS CONDENSER. 
(Model C, Balopticon; Cut loaned by the Bausch & Lamb Optical Co.}. 




Fn;. 34. MAC, ic LANTERN OF THE LATHE-BED TYPE WITH A THREE- 
LENS CONDENSER AND WATER-CELL. 

(Model D, Bal optic on; Cut loaned by the Bausch & Lomb Optical Co.). 



CH. I] 



vSOME AMERICAN MAGIC LANTERNS 



61 




PIG. 35. SECTIONAL VIEW OF AN ARC LAMP AND A TRIPLE-LENS 
CONDENSER WITH WATER-CELL. 

+ W Wire going to the positive carbon. 
W Wire from the negative carbon. 

He Horizontal or upper carbon ; it is positive. 

Vc Vertical or lower carbon ; it is negative. 

L The crater of the positive carbon ; it is the source of light. 

Cond I The first element of the triple-lens condenser. The meniscus is 
always placed with the concavity next the source of light. 

Cond 2 The second element of the triple-lens condenser. It is a plano- 
convex lens and should be of the same focus as the projection objective. The 
different lenses should be in the position shown in this diagram. Between the 
two convex lenses in the parallel beam of light is placed the water-cell. 

B I B 3 Blocks supporting the arc lamp and the condenser. 

Base The base-board with the track along which the different parts move 
(see fig. 40). 

Axis The principal optic axis of the condenser and continuous with that 
of the projection objective. 




FIG. 36 A. MAGIC LANTERN WITH AN AUTOMATIC LAMP AND INCLINED 

CARBONS. 
(Cut loaned by P. Keller & Co., successors to the J. B. Coll Co.). 

This lantern is very widely used. It has a two-lens condenser (see fig. i). 
Its main defect is that every part, lamp, condenser lantern-slide holder and 
objective can be separately raised or lowered. 



62 



SOME AMERICAN MAGIC LANTERNS 



[On. I 




FIG. 36 B. CONDENSER LENS NEXT THE 
RADIANT IN ITS MOUNT. 



This is a'picture of the end of the condenser next the radiant of the lantern 
shown in fig. 36 A. 

The lens is held in place by four thin metal supports, fastened at one end to 
the condenser mount, and hooked over the edge of the condenser at the 
other. The lens is considerably smaller than the condenser mount, thus 
giving abundant room for expansion. 

/, 2, j, 4. The four thin metal strips for holding the lens in position. They 
are white where they hook over the edge of the lens. 

c End view of the metal tube supporting the condenser. 

(The white spots in the condenser face are mirror images of the window's near 
where the picture was taken). 




FIG. 37. MAGIC LANTERN WITH TWO-LENS 
CONDENSER, AND HAND-FEED ARC LAMP. 

(Portable Sciopticon. Cut loaned by the Mclutosh Stereopticon Co.). 



CH. I] 



SOME AMERICAN MAGIC LANTERNS 




FIG. 38 A. SIMPLE MAGIC LANTERN WITH TWO-LENS CONDENSER AND 
A HAND-FEED ARC LAMP WITH RIGHT-ANGLED CARBONS 

AND WATER-CELL. 
(Model 2, Delineascope. Cut loaned by the Spencer Lens Co.). 




FIG. 38 B. DETAILS OF MODEL 2, DELINEASCOPE. 

(Cut loaned by the Spencer Lens Co.). 

The entire instrument is in one metal box. 

At the left is the right-angled arc lamp with the feeding and fine adjustment 
screws. 

The condenser is of the two-lens type with a water cell (W C) between the 
lenses. 

.V P S 1 The slide-carrier is a flat frame on which the slides are laid and 
turned to a vertical position by the crank L. 

S When the crank L turns a slide into position the one already in position 
is released and it falls down the curved incline to S where it can be removed. 

L The projection objective. Its conical holder is hinged so that it can 
be readily turned aside to give place to the projection microscope, which, in 
the figure, is turned over on the top of the lantern box. 



64 



DO AND DO NOT WITH THE MAGIC LANTERN [Cn. I 



99i. Summary of Chapter I : 

Do 

1 . Connect both supply wires 
to the arc lamp as indicated in 
fig. 3, i. e., connect the positive 
wire with the binding post of 
the upper carbon and the nega- 
tive wire with the binding post 
of the lower carbon. 

Make sure of the polarity 
( 80). 

2. Always use a rheostat or 
other balancing device with an 
arc lamp (6). 

3. Insert the rheostat along 
one wire (fig. 1-4). 



4. Insert the ammeter along 
one wire (fig. 2,4). 

5. Always have a double-pole 
switch on the lantern table (fig. 
i-3). 

6. Insert the switch along 
both wires, and before the 
rheostat, so that all the appara- 
tus on the lantern table has no 
current when the switch is open 

(fig. 3). 

7. Always open the switch 
before changing any of the 
wires. 



Do NOT 

i. Do not connect the nega- 
tive wire to the upper carbon 
and thus make the polarity 
wrong. 



2. Never try to use an arc 
lamp without a balancing device 
(rheostat, etc.). 

3 . Do not connect both wires 
with the binding posts of the 
rheostat, but insert it in one 
wire. 

4. Do not connect the am- 
meter with both wires. Insert 
it in one wire. 

5. Do not try to get along 
without a double-pole table 
switch. 

6. Do not insert the switch 
along one wire, but connect it 
with both wires. Do not put 
the switch after the rheostat, 
etc., but before. 

7 . Never change wires on the 
apparatus until the current is 
turned off by opening the 
switch. 



CH. I] DO AND DO NOT WITH THE MAGIC LANTERN 



8. Open the switch before 
inserting or changing carbons. 

9. Center the parts of the 
lantern when it is first installed 
( 51-60). 

10. When the condenser and 
objective are once centered they 
should be fixed in position 



8. Do not try to insert car- 
bons when the current is on. 
Open the switch. 

9-10. After the parts of the 
lantern are once centered, never 
change the position of the con- 
denser or objective for center- 
ing: . 



1 1 . Use the fine adjustments 
on the arc lamp (fig. 3) for cen- 
tering the light on the screen 
after the first centering. Look 
at the carbons through the 
lamp-house window occasionally 
to make sure that they arc in 
the correct relative position 
( 79). 

12. Make sure that the arc 
lamp and condenser, the con- 
denser and objective are separ- 
ated the right distance ( 55- 
56). 

13. For the triple condenser 
select a condenser lens to go 
next the lantern slide which 
shall be of approximately the 
same focus as the projection 
objective, then the light from 
the condenser will cross at the 
center of the objective (fig. 1-2). 



ii. Do not fail to keep the 
light centered by the use of the 
fine adjustments on the lamp 
and by keeping the carbons in 
the correct relative position. 



12. Do not try to use the 
lantern when the arc lamp and 
condenser are too near together 
or too far apart. 

The same for the condenser 
and objective. 

13. Do not try to use an 
objective with a condenser that 
does not cross its rays at the 
center of the objective. Objec- 
tive and condenser should have 
the same focal length approxi- 
mately. 



66 



DO AND DO NOT WITH THE MAGIC LANTERN 



[Cn. I 



14. Make sure that the con- 
denser is arranged with the 
proper lens next the radiant. 
If a three-lens condenser, the 
meniscus should face the source 
of light ; if a two-lens condenser, 
it is the lens in a special mount- 
ing (fig. 36 B), or if there is no 
special mounting, it is the one 
of shorter focus usually, i. e., of 
15 to 19 cm. (6-7^ in.), while 
the one next the objective is 
often of longer focus. 

15. Mark or "spot" the lan- 
tern slides so that they may be 
inserted in the lantern correctly 
( 23, fig. 7, 8, 13) and arrange 
the slides as desired before the 
exhibition ( 21). 

1 6. Make sure that every- 
thing is in working order, the 
room properly darkened, and 
the proper amount of current 
available (10 to 15 amperes). 

1 7 . Light the arc lamp before 
the room lights are turned off 
( 33)- 

1 8. Keep the arc lamp burn- 
ing until the room lights are 
turned on (34). 

19. After the last slide, show 
simply a lighted screen ( 34). 



14. Do not reverse the ends 
of the condenser and thus have 
the wrong lens next the light 
and the wrong one next the 
objective. 



15. Do not try to exhibit 
slides that are not in order and 
not marked for insertion in he 
carrier. 



1 6 . Do not attempt an exhibi- 
tion unless the room is properly 
darkened, and the apparatus in 
working order. 

17. Do not let the room get 
dark, but turn on the arc lamp 
before the room lights are out. 

1 8. Do not turn out the arc 
lamp until the room lights are 
turned on. 

19. Do not keep the last 
slide in the holder too long, but 
show a light screen to indicate 
that the last slide has been 
shown. 



CH. I] DO AND DO NOT WITH THE MAGIC LANTERN 



67 



20. Study the "Troubles," 
their causes and remedies (62- 
98). 

21. Focus the screen image 
sharply, using opera-glasses, if 
necessary (38). 



20. Do not fail to study the 
"Troubles" and their remedies. 

21. Do not let the screen 
image appear vague and out of 
focus. Do not forget the aid 
opera-glasses will give, if the 
screen distance is great. 



CHAPTER II. 

THE MAGIC LANTERN WITH AN ALTERNATING 
CURRENT ARC LAMP AND ITS USE 

100. Apparatus and Material for Chapter II: 

Suitable room with screen (Ch. XII) ; Magic lantern with lan- 
tern table ( 102) ; Arc lamp for alternating current with suitable 
carbons ( 1 08); Alternating current supply ; Rheostat, choke-coil 
or other balancing device ( 105-106); Ammeter for alternating 
current ( in); Incandescent lamp, flash-light, gloves with asbes- 
tos patches, testing lamp, fuses, extra condenser lenses, screw 
driver, pliers, opera-glasses, lantern slides as in Ch. I ( i). 

101. For the historical development of the alternating cur- 
rent arc lamp see the Appendix ; and for the character and advan- 
tages and disadvantages of alternating current see 652-653, and 
modern works on the subject. 

The same books of reference given in 2, Ch. I, are available for 
this chapter. 

COMPARISON OF ALTERNATING AND DIRECT ELECTRIC 
CURRENTS AND LANTERNS 

102. A magic lantern for alternating current may be pre- 
cisely like one for direct current, the only essential difference being 
that the arc lamp must be of the hand-feed type and the mechanism 
for feeding the carbons gives equal movement to the upper and to 
the lower one, both carbons being of the same size. 

One would never use an alternating current with the magic 
lantern if direct current were available. It frequently happens, 
however, that the lighting system of a place is of the alternating 
current type, and no direct current is available. In such a case 
one must make the best of it, or use a motor-generator set or a 
rectifier (see 682-683). 

The objections to an alternating current for the arc lamp in 
projection are: (i) The lamp is noisy; (2) It requires about two 
and one-half times as much current for the same effective light. 

68 



CH. II] ALTERNATING AND DIRECT CURRENT LANTERNS 69 

That is, if 10 to 12 amperes of direct current give satisfactory 
illumination in a given case, it would require from 25 to 30 
amperes of alternating current to give the same brilliancy of 
screen image. Naturally also the heating with the larger alter- 
nating current is greater than with the smaller direct current 
(see also 768). 

103. The difference between direct and alternating current is, 
in general terms, this : the direct current has a constant polarity 
and one carbon is always positive; while the alternating current 
has an alternation of polarity, as the current flows in one direction 
for an instant and then in the opposite direction. The result is 
that each carbon is positive half the time and negative half the 
time, hence both carbons have brilliant craters from which light 
for the screen image might be obtained. Sometimes an effort is 
made to utilize the light from both craters by the arrangement of 
the carbons in the form of a V, the apex of the V pointing toward 
the condenser (fig. 230). 

INSTALLATION OF A MAGIC LANTERN WITH AN ALTERNATING 
CURRENT ARC LIGHT 

104. Wiring from the supply to the lantern. This is pre- 
cisely as for the direct current lamp. If the lantern is to be used 
for experimental purposes it is advantageous to have an incandes- 
cent lamp inserted in the circuit as shown in fig. 2. 

105. Rheostat or other regulating device. There must be 
introduced along one of the supply wires to the lantern some form 
of balancing device. This may be in the form of a rheostat like 
that vised for the direct current ( 6); an inductor or choke-coil, 
a transformer, or a mercury arc rectifier may be used. For the 
special advantages and disadvantages of the different balancing 
devices (see 736-738). 

106. Wiring the lamp. For the alternating current it makes 
no difference which supply wire is connected with the upper carbon, 
as each carbon has an approximately equally brilliant crater. 



70 ALTERNATING AND DIRECT CURRENT LANTERNS [Cn. II 

But in installing a magic lantern for either current, it must 
never be forgotten that the arc lamp must not be connected with the 
main line without some form of rheostat or regulating device in the 
circuit (fig. 3, 40, and 744). 




FIG. 39. MAGIC LANTERN WITH INCLINED CARBONS. 

U C, L C The upper and the lower carbon. Only the carbons of the arc 
lamp are shown. 

A C Alternating current supply wires. 

F Fuses at the outlet box (see fig. 40). 

L Incandescent lamp for use in working around the magic lantern. 

S Double-pole, knife switch. 

R Rheostat in one wire. 

A Ammeter for indicating the amount of current. 

Condenser A two-lens condenser. The light is shown as a parallel beam 
between the lenses. It is usually diverging (see fig. i). 

L S Lantern slide next the condenser. 

Axis Axis The principal optic axis of the condenser and the projection 
objective. 

Objective The projection objective for forming the screen image. 

c Center of the projection objective. The objective and condenser should 
be so related that the light from the condenser crosses at the center when the 
image is in focus on the screen. 

Screen Image The image of the lantern slide on the screen. 

107. Double-pole table switch. This is especially necessary 
when using an alternating current, because with it the current can 
be turned completely off the lamp whenever desired. Any changes 
in the carbons or in the lamp mechanism can then be made with 
safety, as the lamp is completely cut off from the electric supply, 
which would not be the case if a single-pole switch were used. 
The shock from an alternating current supply of no volts is much 



CH. II] ALTERNATING AND DIRECT CURRENT LANTERNS 71 

more disagreeable than from a direct current supply of the same 
voltage. 




FIG. 40. 



MAGIC LANTERN SHOWING THE WIRING AND THE RELATION 
OF THE PARTS. 



Supply Wires Wires from the electric supply to the outlet box. 

Outlet box The iron box receiving the supply wires and containing fuses of 
the cartridge form, a double-pole knife switch and the wires extending to the 
wall receptacle. 

P W R Polarized wall receptacle from which is taken the current to supply 
the arc lamp of the magic lantern. As this receptacle is polarized the cap can 
be put on but one way, and hence the polarity will always be the same if 
the current is direct. With alternating current this form of connection is 
also good. 

Arc Supply The wires extending from the wall receptacle to the table 
switch and the arc lamp. 

Switch The double-pole, knife switch on the lantern table. 

W f The wire extending from the switch to the upper carbon. 

W 2 W 3 Wire from the table switch through the rheostat to the lower 
carbon. 

Arc Lamp Hand-feed, right-angle carbon arc lamp. 

F S Feeding screws for the carbons. 

V A Fine adjustment for moving the source of light vertically. 

L A Fine adjustment for moving the source of light laterally. 

in in Insulation between the carbon holder and the rest of the arc lamp so 
that the current will keep to the carbons instead of short circuiting through the 
lamp. 

5 5 Set screws for holding the carbons in place, etc. 

Lamp-House The metal box enclosing the arc lamp. The feeding and fine 
adjustment screws project through the back end of the lamp-house. 

V Ventilator of the lamp-house. 

Condenser The three-lens condenser. 

Water Cell The vessel of water in the path of the beam. 



72 MAGIC LANTERN WITH ALTERNATING CURRENT [Cn. II 

/ The first element of the condenser consisting in a meniscus lens next the 
arc lamp and a plano-convex lens. 

2 Plano-convex lens toward the lantern slide. The lenses of this condenser 
should be arranged as here shown. 
Objective The projection objective. 

c The optic center where the rays from the condenser should cross when the 
objective is in focus. 

Base Board The board bearing the track and the blocks for supporting the 
different parts. 

Block i, Block 2, Block j The blocks supporting the arc lamp, condenser 
and objective. 

Rods The rods or tubes on the base-board and serving as a track for the 
blocks to move upon. 

108. Arc lamps for alternating current. These are almost 
invariably of the hand-feed type. Lamps are made to hold the 
carbons: (i) at right angles (fig. 1-3); (2) inclined backward 30 
degrees (fig. 23, 39); (3) converging in the form of a V (fig. 23 
D); or (4) even in a vertical position (fig. 22). Each form is 
best adapted to some special purpose. 

With carbons of the same size and composition both carbons 
burn away at the same rate, and therefore must be fed forward at 
the same rate. If the carbons are of different size or material, then 
the mechanism must be adjusted to move the two at a rate which 
shall hold the ends at the same level. 

109. Fine adjustments for the lamp. As indicated for the 
direct current arc lamp ( 10), there should be some means of 
moving one or both carbons separately to compensate for any 
unequal burning. There must also be some means of raising and 
lowering the lamp and moving it sidewisc so that any slight varia- 
tions of the source of light from the axis may be corrected 
( 10, fig. 3). 

110. Lamp-House. There should be a well ventilated metal 
lamp-house of good size and with large doors, so that all the 
apparatus within can be easily got at. There should also be a good 
sized window (say 5 cm., 2 in. square) glazed with smoky mica 
or a combination of green and red glass or some smoked glass of 
sufficient depth of tint for the protection of the eyes. This window 



CH. II] MAGIC LANTERN WITH ALTERNATING CURRENT 73 

should be opposite the craters of the electrodes, so that the position 
of the carbons can be readily seen (fig. 133, 145). 

111. Ammeter for alternating current. The ammeter serves 
the same purpose for the alternating as for the direct current; 
that is, it indicates the amount of current (7). The construction 
for the alternating current is somewhat different, so that the one 
for direct current cannot be used for alternating. On the other 
hand excellent ammeters are now constructed which can be used 
for both alternating and direct currents ( 664, 7o2a). 

112. Mechanical centering in a horizontal axis. This is done 
precisely as for the direct current lantern ( 51, fig. i, 2 and 40) . 

113. Amount of current necessary. In genera) it requires 
from two and one-half to three times as many amperes of alter- 
nating current to get the same brilliancy of image as of the direct 
current (see 755-768). Then for a screen distance of 10 meters 
(30 feet) one should have a current of about 25-30 amperes; and 
for a distance of 15 to 25 meters (50-75 ft.) one should use from 30 
to 45 amperes. If one can be satisfied with less brilliant screen 
images, of course the amount of current may be somewhat less. 

For a further discussion of the comparative merits of direct and 
alternating currents, and means of changing alternating to direct 
current see Ch. XIII, 755-756, 682-683. 

USE OF THE MAGIC LANTERN WITH ALTERNATING CURRENT 
FOR EXHIBITIONS AND LECTURE DEMONSTRATIONS 

114. The suggestions for the lecturer arc as in Chapter I 
( 21-40). 

115. Suggestions for the operator. These arc the same as 
when using the direct current arc lamp ( 26-42), except that in 
using the alternating current arc lamp more care is required to get 
good results. 

(i) The carbons must be properly proportioned to each other. 
If they arc of the same composition they should be of the same size. 
If one is solid and the other cored, the solid one is smaller ( 753a). 



74 MAGIC LANTERN WITH ALTERNATING CURRENT [Cn. II 

(2) As there are two sources of light it is necessary to take 
special pains to focus the lantern slide very sharply on the screen, 
or, when the carbons burn away so that the sources of light are 
relatively far apart, the image on the screen will appear partly 
double like print that has slipped on the press, or like color printing 
when the impressions do not register, thus giving two partly super- 
imposed images, especially if the carbons are arranged like a V. 

If the image is sharply focused and the carbons kept close 
together this trouble will be avoided. 

(3) The carbons must not be allowed to burn away too far 
before they are fed up, or the lantern will become very noisy. The 
carbons should be kept about three mm. (Y% in.) apart. This will 
involve feeding them toward each other every five minutes (see 
also 131, 753a). 

A pair of gloves with asbestos patches (fig. 5) should be at hand 
when working about the alternating current lamp. 

Practically all of the magic lanterns found in the open market 
may be used with an alternating lighting system, provided a lamp 
designed for the alternating current is used ( 102, fig. 3). 

TROUBLES WITH A MAGIC LANTERN WITH ALTERNATING 
CURRENT ARC LAMP 

116. Noisy arc. There is no way of entirely obviating the 
noise in an arc lamp with alternating current. It may be kept at a 
minimum by using carbons of the proper size for the amperage used 
( 753a) and by keeping them relatively close together. As the 
carbons burn away, increasing the length of the arc, the noise 
increases. If a heavy current (much amperage) is used the noise 
becomes very loud and disagreeable. 

The noise is also increased if there is any loose part around the 
rheostat or lamp which can vibrate in unison with the alternations 
of the current. 

117. Managing the arc lamp. Practically all of the arc 

lamps used for the magic lantern with alternating current are of the 
hand-feed type, hence besides all the other things the operator 



CH. II] MAGIC LANTERN WITH ALTERNATING CURRENT 75 

must see to it that the carbons are brought toward each other 
occasionally by turning the proper screws. With moderate cur- 
rents the lamp will run from five to ten minutes without feeding, 
but the greater the amount of current the oftener must the carbons 
be fed together. As stated above, the noise increases with the 
length of the arc ; therefore the carbons should be brought nearer 
together every two to four minutes. 

1 18. Shadows on the screen. All the defects indicated under 
"troubles" in chapter i (83) for the direct current light are liable 
to appear when using alternating current. This is somewhat 
complicated by the presence of an equally brilliant crater on both 
the upper and the lower carbons. As with direct current, there is 
less trouble with right-angled carbons than with vertical or inclined 
ones. With right-angled carbons the defect is greatest when the 
lower carbon is too high, thus shading the upper carbon, as in fig. 
25 A (for the shadows see fig. 24-25, 27-29). As with the direct 
current, the greater the aperture of the projection objective, the 
less marked is the screen defect of a slight mal-position of the car- 
bons. (See also Ch. Ill, 127, Ch. IX, 417, and Ch. X, 488 for 
the arc lamp with small carbons to be used on the house lighting 
system). 



76 SUMMARY FOR ALTERNATING CURRENT LANTERNS [Cn. II 



119. Summary of Chapter II: 
Do 

i. Connect both supply wires 
with the lamp; and remember 
that with the alternating cur- 
rent lamp it makes no difference 
which supply wire goes to the 
binding post of the upper and 
which to the post for the lower 
carbon ( 106). 



2. Insert a rheostat or other 
balancing device along one of 
the supply wires (fig. 3). 



3. Insert the ammeter along 
one wire (fig. 2). 

4. Install a double-pole switch 
before the rheostat (fig. 3). 



5. If the lantern table is on a 
concrete floor, use a board or 
insulating mat to stand on and 
thus avoid possibility of a shock 
if the metal part of the lantern 
is touched ( 98, 689). 

6. Feed the carbons nearer 
together every three to five 
minutes so that the lam]) will 
not be noisy or go out or give 
double screen images. 



Do NOT 

i . Do not fail to connect both 
supply wires to the arc lamp. 



2. Never try to use an arc 
lamp without a rheostat or 
balance. Do not connect the 
rheostat with both, but with a 
single wire. 

3. Do not connect the am- 
meter with both supply wires,' 
but with one. 

4. Do not install a lantern 
without a double-pole, table 
switch which will cut off the 
current from all the apparatus 
on the lantern table (fig. 40). 

5. Do not stand directly on a 
moist concrete floor when oper- 
ating a magic lantern with an 
alternating current lamp. 



6. Do not let the lamp go too 
long before feeding up the car- 
bons. 



CH. II] SUMMARY FOR ALTERNATING CURRENT LANTERNS 77 



7. Focus the screen image 
with special care when using 
alternating current lest the two 
sources of light produce a doub- 
ling of the screen image. 

8. Use opera-glasses, if neces- 
sary, for focusing sharply a 
distant screen image (38). 

9. Look out for shadows on 
the screen. Center carefully 
and remove all causes for shad- 
ows (83-93). 



7. Do not forget the greater 
need for accurate focusing with 
an alternating current lamp, on 
account of the double source of 
light. 

8. Do not forget the advan- 
tage in using opera-glasses for 
focusing if the screen distance 
is great. 

9. Do not permit any defect 
in the management of the lan- 
tern, suspended strings, etc., to 
give shadows on the screen. 



10. Study the "Troubles" in 
116-118, and 62-98. 



10. Do not neglect any of the 
causes for "Troubles." 



CHAPTER III. 

MAGIC LANTERN TO BE USED ON THE HOUSE 
ELECTRIC LIGHTING SYSTEM 

120. Apparatus and Material for Chapter III : 

Suitable room and screen (Ch. XII) ; Magic lantern with lamp- 
house and lantern table; Arc lamp for small carbons ( 127); 
Rheostat ( 129); Flexible cable for connecting the lamp and 
rheostat with the house lighting system (fig. 40) ; Separable plugs 
and extension plugs (fig. 49-50); Polarized plugs (fig. 48-49); 
Nernst lamps (fig. 54-55); Objective shield (fig. 14); Concen- 
trated filament, Mazda lamps ( 136) ; Flash-light; testing lamp, 
screw drivers and pliers; lantern slides, etc., as in Ch. I. 

121. For the historical summary of the use of the house, 
electric lighting system for the magic lantern, see the Appendix. 

For works of reference see 2. Consult also the Microscopical 
Journals, and the catalogues of manufacturers of projection 
apparatus. 



MAGIC LANTERN WITH SMALL CURRENT ELECTRIC LIGHTS FOR 
LABORATORY AND HOME USE 

122. For public exhibitions and large lecture rooms special 
electric wiring and large current arc lamps are necessary, as 
described in Ch. I, II and XIII. For small audiences as in labora- 
tories and for home use, where less than 100 people are usually 
present, very satisfactory results may be obtained by means of 
lighting apparatus drawing current from the ordinary house light- 
ing system ; and the electric current may be direct or alternating. 

123. Kinds of lamps to be used with small currents. There 
are three forms of lamps which have been successful for use with 
the magic lantern drawing current from an ordinary lighting 
system : 

(i) An arc lamp of small size using small carbons, i. e. carbons 
of 6 to 8 mm. (% to r /i<; in.) in diameter. A large arc lamp 
is equally available if it has long clamping screws, bushings or 

78 



CH. Ill] MAGIC LANTERN WITH SMALL CURRENTS 



79 




FIG. 41. THE LILIPUT ARC LAMP OF LEITZ. 

This lamp was designed to use with the Edinger drawing apparatus and with 
the condenser for dark ground illumination, etc. Both carbons are moved 
equally by means of the rack and pinion movement. For direct current the 
horizontal or positive carbon is larger than the vertical or negative carbon in 
the proportion of 8 to 6. 

The condensing lens in the tube is mounted in a telescoping sleeve. When 
the sleeve is in, the lens is at its principal focal distance from the crater, and 
gives a parallel beam of light. When the sleeve is pulled out more or less the 
condenser gives a converging beam of light. 

For use with the magic lantern the tube and special condenser are removed, 
as shown in fig. 47. 

adapters for the small carbons. Such carbons require from three 
to six amperes of current for the best effect (fig. 41-44). 

(2) A Nernst lamp with one or more filaments (fig. 54-55). 

(3) A Mazda lamp with concentrated filament (fig. 52). 

The arc lamp is permanent. One has simply to renew the 
carbons when they are burned out. 

If alternating current is used, carbons 150 mm. (6 in.) long and 
8 mm. ( 5/ i<5 in.) in diameter last about three hours. 

If direct current is used the upper carbon is 8 mm. ( 5 /ir, in.) 
and the lower carbon 6 mm. (^4 in.) in diameter. Both are 150 
rrm. (6 in.) long, and they last about three hours ( 753a). 

The Nernst and Mazda lamps are fragile and must be handled 
carefully. They have a working life of 500 hours, more or less, 
then a new lamp must be obtained. 



So MAGIC LANTERN WITH SMALL CURRENTS [Cn. Ill 

124. Room for projection. Any room may be used at night, 
and this makes these magic lanterns especially adapted for the 
home. 

In the daytime, of course, the room where they are used must 
have shutters or curtains so that it can be darkened. 

125. Screen for the image. The screen need not be over 
three or four meters square (9-1 2 feet) . For many purposes a large 
sheet of cardboard, 72x120 cm. (28x44 in-) makes the best 
possible screen (see Ch. XII). 

For home use a white wall or a well stretched sheet will serve. 
If the screen is to be used frequently in the same place in the 
laboratory or home it is desirable to use a white wall or a regularly 
painted screen (see 621-630). 

126. The magic lantern and its support. Any of the good 
modern forms of magic lantern can be used. Special small and 
compact lanterns have been constructed for this purpose, and they 
are excellent and cheap (see prices in the appendix) (fig. 51-52). 

For a lantern support any table of sufficient height may be used. 
A pile of books or an empty box on an ordinary table will serve 
to raise the lantern sufficiently. 

ARC LAMPS FOR THE HOUSE CIRCUIT 

127. Small arc lamps, using small carbons only, are con- 
venient ; but the ordinary large arc lamp can be used if the screws 
for clamping the carbons are long enough, or by means of bushings 
or adapters for the small carbons (for wiring and rheostat, see 
128-129). 

The small carbon arc lamps are easily managed, and the amount 
of light they give (see 756) much more than offsets the attention 
they require over the other lamps used on the house circuit. 

If a lamp must be purchased for use on the house circuit, one of 
small size is preferable. They arc designed for the small carbons 
only. They arc nearly always of the hand-feed type, but when 
direct current is available there arc automatic lamps to be had. 
The Thompson automatic arc lamp, and the Bausch & Lomb 



CH. Ill] MAGIC LANTERN WITH SMALL CURRENTS 81 




FIG. 42. THE SMALL ARC LAMP OF THE SPENCER LENS Co. 

With this small arc lamp the two carbons may be moved separately or 
together, as the carbon movement is like that of the larger lamps, i. e., one shaft 
within the other, and the corresponding milled heads are placed close together, 
so that either can be turned separately or both together. 

It is arranged for giving parallel or converging light. When used with the 
magic lantern the special condenser and its tube are removed (fig. 47). 

automatic lamp are so adjusted, or may be so adjusted if desired, 
that they will work with currents ranging from 5 to 25 amperes 

(fig. 41-44)- 

The small lamps (from their size, called "Liliput or baby" arc 
lamps) are largely used for darkground illumination and ultra- 
microscopy and for drawing. For these purposes they have a tube 
attached with a condensing lens (fig. 41). For use with the magic 
lantern the tube and condensing lens are removed (fig. i). 



82 



ARC LAMPS WITH SMALL CURRENTS [Cn. Ill 




FIG. 43. THE SMALL ARC LAMP OF REICHERT. 

This is arranged in the figure for giving a parallel beam of light from the 
small condenser; and the mechanism for feeding the carbons can he actuated 
at a distance by means of a Hooke's joint and rod. 
The horizontal or positive carbon. 

Clamp for holding the lamp to the upright at any desired height. 
Milled head of the feeding mechanism for the carbons. 
Rod extending from the Hooke's joint. 
e - ?,> f- S- Holders and clamping screws for the carbons. 
/ Terminal points of the carbons where the arc is formed. 
ra The tube holding the condensing lens. It is cut away on one side to 
show the carbons. 

k' ' n The condensing lens in the end of the tube. It is at the principal focal 
distance from the crater and the diverging beam is made parallel; by pulling 
it to the right the beam will be converging. 



WIRING AND CONNECTING THE ARC LAMP WITH THE HOUSE 

CIRCUIT 

128. Wiring. The wiring is in principle exactly as for the 
large current arc lamp (fig. 1,2, 45). 

One end of a double, flexible cable of sufficient length (2 meters, 
6 ft. at least) is connected with a separable attachment plug (fig. 
49). The two wires near the other end of the cable arc separated 
for a short distance, and one wire is cut. The cut ends of this wire 



CH. Ill] 



ARC LAMPS WITH SMALL CURRENTS 



are then inserted into the binding posts of the rheostat (fig. 45). 
This puts the rheostat along one supply wire (in series) . 

The cut ends of the cable are then connected with the binding 
posts of the arc lamp (fig. 45). For polarity see 701. 

129. Rheostat or other balancing device. As with the arc 
lamp for heavy currents, those to be used on the house circuit must 
also have a balancing device of some sort like a rheostat. It must 
be in one wire (fig. 45). 

Never try to use an arc lamp on any circuit without a rheostat 
or other balancing device. If one is not used the fuses will be 
burned out. 




FIG. 44. REICHERT'S AUTOMATIC ARC LAMP FOR USE ox THE HOUSE 
LIGHTING SYSTEM IF DIRECT CURRENT is AVAILABLE. 

At the bottom are screws for fine adjustment, laterally or vertically. 



128a. In modern wiring for incandescent lamps each group of not over 
1 6, or in special cases not over 32, lamp sockets must be protected by a fuse 
or cut-out. The wire must be equivalent to a copper wire No. 14 or No. 18 
B. & S. gauge, and the fuse or cut-out must be for not over 10 amperes (usually 
6 amperes) for a 1 10 volt circuit. This is sufficient for the small arc lamp. 

In the older constructions where only one to three lamps were on a single 
line, very weak fuses were used which would melt if over two or three amperes 
were drawn from the line. Naturally, on a house circuit thus wired and fused, 
the fuses would be burned out if one tried to use the small arc lamp upon it, for 
that rarely draws less than four amperes and often as many as six. 

In using the arc lamp on the house circuit it is therefore necessary to make 
sure that the wiring and fuses arc of sufficient capacity for the current needed. 



8 4 



ARC LAMPS WITH SMALL CURRENTS 



[CH. Ill 



The rheostat needed for the small-current, arc lamp is small and 
inexpensive. It need not be adjustable. One has only to be cer- 
tain that it will not deliver a current above five or six amperes. 

In purchasing a rheostat for the house circuit, tell the manufac- 
turer the kind of current (direct or alternating) and the voltage 
(no or 220). If one does not know the character and voltage of 
his house circuit the information can be obtained at the office of 
the company furnishing the current. 



Lamp Socket s P 




FIG. 45. WIRING AND CONNECTIONS OF THE ARC LAMP USED ON THE 
HOUSE LIGHTING SYSTEM. 

130. Polarity with the arc lamp. With alternating current 
both wires are the same (see 103 and 653), but with direct current 
one of the wires is positive and one negative, and the positive wire 
must be connected with the binding post for the upper carbon. 
The most practical ways of determining the polarity are described in 
Ch. I, 80; Ch. XIII, 702. 

In case the lower carbon shows the brightest crater it is positive 
and hence the polarity wrong. If the separable attachment plug is 
of the polarized form, separate the two parts thus turning off the 
current. Then reverse the position of the wires in the binding 
posts of the lamp. This will connect the positive wire with the 
upper carbon as it should be. A simple way, if non-polarized plugs 



CH. Ill] 



ARC LAMPS WITH SMALL CURRENTS 



are used (fig. 496), is to leave the wires as they are in thelamp.but 
pull the separable plug apart and turn it half way round. This will 
reverse the position of the connections so that the polarity will be 
found correct on lighting the lamp again. 

When the correct polarity has been obtained at one particular 
lamp socket it is well to make a straight line with a glass pencil, a 
pen or a brush across the socket, and the two parts of the separable 
plug, then the correct connections can be made with that socket at 
any time without trouble. 



Arc Lamp 




KS 



FIG. 46. THE MAGIC LANTERN FOR USE ON THE HOUSE LIGHTING 

SYSTEM. 

SW Supply wires to the lamp socket (So). 

So, K The lamp socket with the key switch. 

S P Separable attachment plug. The cap has been removed to show 
the metal prongs serving to make the contact. 

L W Wires connecting the cap of the separable plug with the knife switch. 
(K S). As shown in fig. 45, 47, the knife switch is more frequently omitted. 

K S Double-pole knife switch for opening and closing the circuit. 

Rheostat For controlling the current. It is in one wire. 

Arc Lamp This is one of the small forms. 

5 5 Set screws for holding the carbons in place. 

h c Horizontal or upper carbon. 

v c Vertical or lower carbon. 

In In Insulation between the carbon holders and the rest of the lamp to 
compel the current to follow the carbons, and not to short circuit. 

fs Feeding screws for moving the carbons. 

cl Clamp to fix the lamp at any desired position on the vertical rod. 

Condenser The two-lens condenser for illuminating the lantern slide. 

i 2 The two plano-convex lenses with their curved surfaces facing each 
other. 

L S Lantern slide close to the condenser. 

Axis Axis The principal optic axis of the condenser and the objective. 

Objective The projection objective for giving the screen image. 

Image Screen The white screen on which the image is projected. 



86 



ARC LAMPS WITH SMALL CURRENTS 



[CH. Ill 



Condenser 




FIG. 47. THE MAGIC LANTERN WITH A THREE-LENS CONDENSER AND A 
WATER-CELL FOR USE ON THE HOUSE LIGHTING SYSTEM. 

This is the same as fig. 46 except that no double-pole knife switch is used, and 
there is a triple-lens condenser and water-cell in place of a double-lens condenser. 

It is well also, when one has the lamp properly connected, to turn 
off the current by opening the separable plug, and then paint the 
positive wire red where it is inserted into the binding post for the 
upper carbon. The negative wire can be painted black also. If 





HUBBEuOf 



FIG. 48. WALL RECEPTACLES WITH SEPARABLE CAP. 
(Cuts loaned by II. Ilubbell, Inc.). 

A Wall receptacle with the connecting prongs polarized so that the cap 
can be put on only one way, thus avoiding change of polarity with direct cur- 
rent. 

B Wall receptacle in which the cap can be put in place either way around. 

Either form can be used with both direct and alternating current. 



CH. Ill] 



ARC LAMPS WITH SMALL CURRENTS 



these precautions are taken, it will be very simple to connect up 
the lamp correctly at any time. 

"Polarized attachment and extension plugs" are made (fig. 48A, 
4pA). These can only be put together one way. They are very 
convenient for direct current connections; they are also equally 
adapted for alternating current. 





FIG. 49. SEPARABLE ATTACHMENT PLUGS. 
(Cuts loaned by II. Ilubbell, Inc.}. 

A Polarized, separable plug for a lamp socket. The metal prongs are in 
planes at right angles and hence can be inserted in only one way, thus avoiding 
change of polarity with direct current. 

B Non-polarized attachment plug. The connection can be made either 
way around as the prongs are in the same plane. 




FIG. 50. SEPARABLE EXTENSION CONNECTOR. 
(Cut loaned by II. Ilubbell, Inc.). 

This is to enable one to extend the line by joining separate cables. These 
extension connectors can be had with polarized or non-polarized prongs to the 
cap. 

131. Carbons for small currents; feeding the carbons. For 

the small currents used with the house circuit, the carbons should 
be small. For alternating current of five to six amperes, 8 mm. 
carbons answer well. For three to four amperes the carbons 
should not be over 6 mm. in diameter. 



ARC LAMPS WITH SMALL CURRENTS [Cn. Ill 

For direct current the two carbons must be of different size if the 
feeding mechanism of the lamp moves the carbons equally. With 
an equal feeding mechanism, the upper or positive carbon can be 
7 mm., the lower one 5 mm., or the upper 8 mm. and the lower 
one 6 mm. 

One could use carbons of the same diameter for direct current, 
but it would be necessary to feed the upper or positive one more 
rapidly than the lower one on account of the unequal rate of burn- 
ing, otherwise the correct relative position of the carbons would 
not be maintained (fig. 24-25). On a no volt, direct current cir- 
cuit, the lamp will burn about six minutes without going out. 
The carbons should be fed up every three to five minutes. 

For alternating current of no volts, the small lamps will burn 
from eight to ten minutes, sometimes longer. It is well to feed the 
carbons every five to seven minutes. 

In case a choke-coil is used (Ch. XIII, 736), the lamp burns 
more quietly and will burn longer without being fed. If a step- 
down transformer is used, then the right-angled lamp will not burn 
so long only one to two minutes while a lamp with inclined 
carbons will burn three minutes, because it takes a higher voltage 
to maintain the right-angled than the inclined carbon arc (see Ch. 
XIII, 753, 768). 



TURNING THE ARC LAMP ON AND OFF 

132. Lighting the small arc lamp. For this, make sure that 
the carbons arc not in contact. Now turn the switch for the room 
lights and the snap switch in the socket where the separable attach- 
ment plug for the lamp wiring is screwed in. Feed the carbons 
together until they touch. There should be a flash of light. 
Separate the carbons two or three millimeters as soon as the flash 
is seen and the arc will be established and the light will be at full 
brilliance. Sometimes it is necessary to keep the carbons almost 
in contact for a half minute or so, until the tips arc well heated, 
before the arc will bum. If on separating the carbons the light 
goes out, they must be brought together again as at first. 



CH. Ill] ARC LAMPS WITH SMALL CURRENTS 89 

133. Turning off the small arc lamp. The snap or key switch 
in the usual incandescent lamp socket is designed to break the cir- 
cuit where, at most, two amperes are used. These key switches, if 
used to interrupt a relatively large current, like that used for the 
small arc lamp, are liable to start an arc within the socket. If such 
an arc is started, the socket will be short circuited, resulting either 
in the burning out of a fuse, the burning out of the socket or some- 
thing more serious. 

The liability of a socket to arc is much greater with direct than 
with alternating current. The liability to arc is also much greater 
if the key switch is turned slowly than when it is turned quickly. 

By observing the following directions the current may be turned 
off with perfect safety : 

(1) Turn off the current by separating the carbons until the 
lamp goes out, then the key switch may be used, or a plug or exten- 
sion pulled apart. 

(2) Turn off the current by pulling the separable plug or the 
separable extension apart (fig. 49-50). 

(3) Make use of a knife- or snap-switch (fig. 1,2, 40). 

(4) Do not turn off the current by the key switch in the bulb 
socket. When the lamp is out, it is safe to turn the key switch in 
the socket. 

(5) Do not unscrew a plug to turn off the light, for the 
break in the circuit is so slow that an arc will almost certainly 
be formed. 

134. What to do in case the key switch is used and an arc 
is formed in the socket: 

(1) Turn the key on again as quickly as possible. 

(2) If the arc lamp is still burning after turning on the key 
switch, turn the lamp off by method i to 3 ( 133). 

(3) Go to the nearest room switch and turn off the current. 

In case a fuse is blown out which is almost sure to occur if an 
arc is formed in the socket or if the lamp socket is burned out, it 
is wise to call in an electrician to make the necessary repairs. 
This, of course, assumes that the user has not the technical knowl- 
edge necessary to make the corrections himself. It is further 



QO MAGIC LANTERNS WITH MAZDA LAMPS [Cn. Ill 

assumed that if he had possessed the technical knowledge no mis- 
takes, and hence no accident would have happened. 

135. Use of the small arc lamp for demonstrations and 
exhibitions. The centering of the apparatus to one axis, and 
using the correctly proportioned condenser and projection objec- 
tive, the lighting and putting out the lamp, arrangement and 
insertion of lantern slides, etc., are all exactly as described in Ch. 
I, II ( 26-41, 52, 112). 




FIG. 51. MAGIC LANTERN WITH SMALL ARC LAMP. 
(Balopticon B.; Cut loaned by the Bausch & Lomb Optical Co.). 

MAGIC LANTERN WITH A MAZDA, CONCENTRATED FILAMENT 
INCANDESCENT LAMP 

136. Next to the arc lamp the Mazda concentrated filament 
lamp is perhaps the best electric light at present available. They 
arc as simple to use as an ordinary incandescent bulb. No rheo- 
stat is necessary. The lamp is on a stand by which it may be 
raised and lowered and brought the proper distance from the con- 
denser (fig. 52-53). 

137. Connections with the house circuit. This is made by a 
double flexible cable, one end of which is connected with a separable 
plug, and the other with the lam]) socket of the Mazda lamp. As 
no rheostat is used, and as the light is turned on and off exactly as 
for any incandescent bulb, this light is absolutely simple in use. 
It gives a light sufficient for a small room, where not over 50 to 
IDC people are to watch the exhibition. 



CH. Ill] MAGIC LANTERNS WITH MAZDA LAMPS 91 




FIG. 52. SIMPLE MAGIC LANTERN WITH INCANDESCENT 
LAMP AS RADIANT. 

(Cut loaned by Williams, Brown & Earle). 

This is known as the "Society Incandescent Lantern No. 3 G." It is 
especially designed for use with permanently mounted lantern slides (fig. 15). 

138. Centering and distance from the condenser. The 

centering along one axis is as with the arc lamp ( 51). 

In general the concentrated filament should be at the principal 
focal distance from the condenser. One can determine the best 
position by the use of a good lantern slide and changing the dis- 
tance of light and condenser until the best position is found. It is 
well to mark that position for future use. 




FIG. 53. MAGIC LANTERN WITH MAZDA LAMP. 
(Balopticon B.; Cut loaned by the Bausch 6" Lornb Optical Co.}. 

This lantern can be used with the small arc lamp on the house lighting sys- 
tem, with the Mazda incandescent lamp or with acetylene. 



92 MAGIC LANTERN WITH NERNST LIGHT [Cn. Ill 

^139. Management of an exhibition with the Mazda lamp. 

The exhibition should be managed as for the arc light ( 26-41). 

One must remember that with this relatively weak light only a 
small screen image should be attempted, and that the room must 
be relatively darker than for the arc light. In brilliancy the screen 
images will be more like that of the old lanternists with their weak 
lights. Clear lantern slides are especially desirable. The very 
opaque lantern slides sometimes met with can only be well shown 
by a large arc lamp. 

MAGIC LANTERN WITH A NERNST AUTOMATIC LAMP 

140. This is also an excellent lamp to use with a magic lantern 
in a small room. Some forms are automatic in starting when the 
current is turned on, and some have to be specially heated. The 
automatic form is to be preferred, for it is no more trouble to run 
than an ordinary incandescent lamp. It takes some time, usually 
one to three minutes, for the glowers to come to full brilliancy after 
the current is turned on. They are made for the lantern with one, 
two, three and four filaments or glowers. The single glower 
approximates most closely to the arc lamp in the smallness of the 
source of light. Of course, with the multiple glower lamps a greater 
amount of light is given off, but they make an extended source. 
Whether the lamp has one or more filaments it can be attached 
directly to the house lighting system through any incandescent 
bulb socket as described for the Mazda lamp ( 137). 

141. Rheostat or ballast for the Nernst lamp. This lamp 
like the arc lamp is always used with a balancing device, but unlike 
the arc lam]), the ballast is an integral part of the lamp as pur- 
chased, and not a separate apparatus as with the arc light (fig. 54, 
55 and i). 

The glowers and the ballast must be adapted to each other 
and both must be adapted to the line voltage. 

The ballasts, which are enclosed in a vacuum glass, as with an 
incandescent bulb, sometimes burn out. The filaments will not 
then glow when the current is turned on. If a ballast burns out it 
must be replaced by a perfect one. 



CH. Ill] MAGIC LANTERN WITH NERNST LIGHT 



93 



WI 



W2 




FIG. 54. NERNST LAMP FOR THE MAGIC LANTERN (REICHERT). 

This is on a support and has a rack and pinion for raising and lowering the 
lamp. It is automatic. 

Gl The three filaments or glowers with this lamp. 
S 3^ The two supply wires from the house circuit. 
T t Pinion and milled head of the rack work. 
W It W 2 , W 3 The three ballast tubes. 

142. Centering and distance from the condenser with the 
Nernst lamp. The lamp must be centered with the condenser and 
objective as described in Ch. I ( 51+). It must have some form 
of support with means of raising and lowering the lamp. The 
distance of the lamp from the condenser which gives the best 
illumination can be determined as follows: Light the lamp, put a 
good lantern slide in the lantern, then move the lamp up toward 
the condenser, shifting it back and forth until the best screen image 
is produced. In general, it will be found that this results when the 
glower is at about the principal focal distance from the condenser. 
When the best position is found the place should be clearly marked, 
then the lamp can be put in this position quickly at any time. 

143. Connecting the lamp with the house circuit ; alternating 
current. This is done by means of a flexible conductor connected 



94 



MAGIC LANTERN WITH NERNST LIGHT 



[On. Ill 



with the lamp at one end, and a separable plug at the other (fig. 
49 A). The plug is of standard size and can be screwed into the 
socket or receptacle of any incandescent lamp. 

To light the lamp turn the key switch of the socket as for an 
incandescent lamp, and in a minute or longer the glower or glowers 
will attain their full brilliancy, and one can use the lamp as long 
as desired without further attention. 




FIG. 55. NERXST LAMP OR SCHWANN-LIGHT. 

(Cut loaned by the Chas. Beslcr Co.}. 

If one uses a three or four glower lamp drawing about four 
amperes of current there might be a short circuit in the incandes- 
cent lamp socket if the snap switch were turned off slowly. If that 
is used, turn it as quickly as possible (see 133). Pulling the 
separable attachment plug apart will avoid all danger ( 133). 

144. Nernst lamp and direct current. If one has a direct 
current lighting system, then the Nernst lam]) must be adapted to 
that, and must be connected properly as with the direct current arc 
lam]). The two connections with the lam]) are marked, plus (-{-) 
and minus ( ); or positive (P) and negative (N); and the corre- 



CH. Ill] MAGIC LANTERN WITH NERNST LIGHT 95 

spending wires must be attached to these lamp binding posts or 
connections, or the lamp will soon burn out. Unfortunately, one 
cannot tell by simple observation when the wires are connected 
properly, as for the arc lamp ; but he must determine the polarity 
of the wires before connecting them with the lamp (see Ch. XIII, 
701-703)- 

145. Marking the wires and attachments after determining 
the polarity with the direct current system. When the polarity of 
the wires is determined, if one is to use the same place for current 
repeatedly, it is a good plan to mark the position of the socket and 
the two parts of the separable plug by a straight line of colored 
paint when all are in position. Then it will be easy to connect up 
the parts correctly at any future time. Then if the positive wire 
has its insulation material colored red, at least at the lamp end, it 
will enable one to connect up with that particular socket correctly 
at any future time. It is also a convenience to have polarized 
separable plugs (fig. 4pA) , then the two parts of the plug cannot be 
reversed if they should become separated. On the other hand, if 
the attached part is left in place and the cap or removable part 
pulled off, one can make the connection correctly at any time with- 
out trouble, as it cannot be put together wrong. 

Unfortunately, one cannot be sure that a separable plug and the 
lead wires connected to the lamp properly for one incandescent 
socket will be so for any other, and one must determine the polarity 
for each socket. 

An alternating current is more satisfactory for the Nernst lamp 
than a direct current, for with the alternating current one does not 
have to trouble about the polarity of the two wires, since both are 
alike. 

146. Management of an exhibition with the Nernst light. 

This is precisely as for any other magic lantern radiant except that 
the lamp must be started three to four minutes before it is needed, 
for it may take that time to get good illumination. Furthermore, 
it is better to leave the light burning during the entire lecture, so 
that there will be no delays. The light can be shut off the screen 
during the intervals with the objective shield (fig. 14). 



96 TROUBLES WITH SMALL CURRENT LANTERNS [Cn. Ill 

147. Troubles with the magic lantern on the house lighting 
system. With the arc lamp these are the same as those indicated 
in Ch. I, 62-98; Ch. II, 116-118. See also i28a for fuses. 
There is also the danger of starting an arc in the incandescent 
socket from which the current is drawn unless the precautions given 
in 133 for turning out the light are observed. 

For all the lights the management of the exhibition, centering of 
apparatus, etc., are the same as for the lanterns in Ch. I, II. 

The most striking difficulty will probably be the comparatively 
dim screen pictures as compared with the brilliant screen images 
given by the large current arc lamp. 

The room must be darker and the screen picture smaller with 
these lights. 

The Mazda lamp may go out on account of the breakage of some 
of the connections within the bulb. If this happens the only thing 
to do is to use a new lamp. It is wise to have several on hand. 

With the Nernst lamp also some of the connections are liable to 
break, or the ballast may burn out, or the glower be broken. 
Usually only the defective parts must be renewed, and not an 
entirely new lamp obtained. 



CH. Ill] SUMMARY FOR SMALL CURRENT LIGHTS 



97 



148. Summary of Chapter III: 



Do 

1. Find out the kind of cur- 
rent used in the house lighting 
system and the voltage (alter- 
nating or direct current; vol- 
tage no or 220). 

2. Wire the small arc lamp 
exactly as the large arc lamp is 
wired (fig. 40). 

3. Always use a rheostat or 
some other balancing device 
with the arc lamp ( 129). 

4. Use small carbons for the 
arc lamp on the house circuit 
(131)- 

5. Make sure of the polarity 
if direct current is used (701- 
73)- 



6. Follow carefully the direc- 
tions for lighting the arc lamp 
( J 32). 

7. Be very careful to turn off 
the arc lamp by one of the safe 
methods ( 133). 

8. Make the room darker for 
the small arc lamp than for the 
large one, and have a smaller 
screen picture ( 139). 



Do NOT 

1. Do not try to use an arc 
lamp on the house circuit with- 
out knowing the kind of current 
and the voltage. 

2 . Do not wire the small lamp 
differently from the large lamp 
except that smaller wire can be 
used. 

3. Never use the arc lamp 
without a proper balancing 
device. 

4. Do not use large carbons 
for the lamp on the house cir- 
cuit, they would not heat 
enough to give a good light. 

5. Do not worry about the 
polarity if alternating current 
is used. If direct current is 
used the polarity must be 
attended to so that the upper 
carbon is positive. 

6. Do not have the carbons 
in contact when turning on the 
current. 

7. Do not turn off the arc 
lamp by the socket switch. 

8. Do not expect so much of 
the small as of the large current 
arc lamp. Do not have the 
room too light for the small 
lamp. 



SUMMARY FOR SMALL CURRENT LIGHTS [Cn. Ill 



Do 

1. Wire for the Mazda lamp 
exactly as for any incandescent 
bulb lamp. 

2. Turn the lamp on and off 
by the key switch as for any 
incandescent lamp. 

3. As this light is relatively 
dim, make the room dark and 
project a small picture. 

It is also wise to have one or 
more extra bulbs in case one 
burns out. 



Do NOT 

1 . Do not use a rheostat with 
a Mazda lamp. 

2. Do not take any more 
trouble with the concentrated 
filament Mazda than for any 
bulb lamp. 

3. Do not try to make too 
large a screen picture; and do 
not have the room as light as 
for the arc lamp. 



4. Turn the lamp on and off 
whenever desired as it gives full 
brilliancy in an instant. 



4. Do not let the lamp burn 
all the time during an inter- 
mittent exhibition any more 
than with the arc lamp. 



Do 

1. Find out the kind of cur- 
rent and the voltage wherever a 
Nernst lamp is to be used. 

2. Purchase a Nernst lamp 
adapted to the current with 
which it must be used. 



Do NOT 

1. Do not use a Nernst lamp 
with a current and voltage for 
which it was not constructed. 

2. Do not purchase a Nernst 
lamp for direct current if it 
must be used on an alternating 
current line. 



3. A special rheostat or bal- 
last forms a part of ever) 7 
Nernst lamp for projection. 



3. Do not insert a separate 
rheostat in the wiring for a 
Nernst lamp. 



CH. Ill] SUMMARY FOR SMALL CURRENT LIGHTS 



99 



4. Wire the Nernst lamp just 
as the arc lamp is wired except 
that no separate rheostat is 
inserted. Wire the Nernst 
lamp for alternating current 
just as a Mazda incandescent 
lamp is wired. 

5. Wire the Nernst lamp for 
direct current with the positive 
wire in the binding post marked 
+ or P, i. e., the same as a 
direct current arc lamp is wired, 
except that no separate rheostat 
is included ( 141). 

6. Determine the polarity of 
the supply wires with precision 
and care ( 701-703). 

7. Let the Nernst lamp burn 
during the entire exhibition, as 
it takes from one to three 
minutes for the light to reach 
full brilliancy. 

8. Shut the light off the 
screen when not needed, by the 
objective shield (fig. 14). 

9. Handle the Nernst lamp 
carefully, as it is easily injured. 

10. Manage the exhibition 
with a Nernst lamp as with any 
other light, remembering the 
need of a dark room and a screen 
picture of moderate size for this 
relativelv weak light. 



4. Do not worry about polar- 
ity in wiring the Nernst lamp 
for an alternating current sys- 
tem. 



5. For a direct current cir- 
cuit, do not put the positive 
wire in the negative binding 
post of the Nernst lamp. 



6. Do not neglect the polarity 
of the two wires on a direct 
current circuit. 

7. Do not turn the Nernst 
lamp out during an exhibition 
for it takes too long to light it. 



8. Do not forget to use the 
objective shield for shutting the 
light off the screen when it is 
not needed. 

9. Do not handle the Nernst 
lamp roughly. It is delicate. 

10. Do not expect too much 
of a Nernst lamp with the magic 
lantern. One cannot have the 
room so light, nor project such 
large screen pictures, nor use 
such dark lantern slides as with 
the arc lamp. 



CHAPTER IV 

THE MAGIC LANTERN WITH THE LIME LIGHT 
AND ITS USE 

150. Apparatus and material for Chapter IV : 

Suitable room with screen (Ch. XII) ; Magic lantern with a suit- 
able lamp-house and a lime-light burner (fig. 56-59) ; Cylinders 
of compressed Oxygen and Hydrogen ( 154-155); Lime or other 
refractory substance for giving the light ( 153, 157); Oxygen 
generator and ether saturator ( 177-179); Objective shield (fig. 
14, 62, 169); Tubes for making the connections ( 159, i59a); 
Flash-light, screw drivers and pliers, asbestos-patch gloves (fig. 61) ; 
lantern slides, etc.; Matches or gas lighters ( 160). 

151. For the discovery that oxygen and hydrogen burning 
together give a very hot flame, and that dazzling light is produced 
by directing the flame against lime, etc., and the application to the 
magic lantern, see the Appendix. 

For works of reference see Chapter I, 2. 

THE LIME LIGHT FOR THE MAGIC LANTERN 

152. The Magic Lantern used with the lime light is in every 
way like the standard magic lantern with the direct current arc 
lamp with the single difference of the source of light. 

153. The lime light. This is one of the most brilliant avail- 
able lights for projection purposes. It is produced by directing 
the exceedingly hot flame of hydrogen burning in oxygen against a 
piece of unslaked lime. The oxy -hydrogen flame in itself is not 
brilliant, but the heated lime gives a light of dazzling brilliancy 
from a very small area ; hence it is especially well adapted for pro- 
jection with the magic lantern and the projection microscope. 

If the candle-power of a lime light is compared with the other 
lights used for projection it will be seen that it stands third, sun- 
light being first and the arc light second. 

Hydrogen is not always used, but illuminating gas, the vapor of 
alcohol, ether or gasoline sometimes takes its place. 

Unslaked lime is not the only refractory substance which gives 
great incandescence. Zirconium discs and discs made of the mix- 



CH. IV] MAGIC LANTERN WITH THE LIME LIGHT 101 

ture of thorium and cereum such as is used in Welsbach mantles 
have been employed. Nothing gives a more brilliant incandescence 
than the unslaked lime, but it deteriorates rapidly by absorbing 
moisture when exposed to the air. This is not the case with 7/ircon 
and thorium ; the discs of these may be used over and over, some- 
times hundreds of times, while with the limes one usually has to put 
a new one in place every time the lantern is used ( 153 a). 




FIG. 56. MAGIC LANTERN WITH THE 
LIME LIGHT. 

(From the Catalogue of the Enterprise Opt. Mfg. Co.). 

The door of the lamp-house is open, showing the burner with the lime in 
position. 

H The hydrogen supply tube, extending to the burner. 
The oxygen supply tube, extending to the burner. 

154. Oxygen gas in steel cylinders. This is now a great 
article of commerce. Nearly every large drug store keeps one or 
more of them in stock for the use of physicians. The steel cylin- 
ders for containing oxygen were formerly large and contained 
oxygen under a pressure of about 17 atmospheres (250 pounds per 
square inch). Such cylinders are still used; but at the present 

153a. There has lately been introduced a substitute for limes, known as 
Guil Pastils. These are rather soft white cylinders of a substance giving 
great brilliancy when used in place of lime. The Guil pastil is put into the 
holder so that the end is heated, hence the lamp should be in the form shown 
in fig._ 57 K, not as in fig. 56 or 59 L. The Guil pastil serves for 10 to 20 
exhibitions. It is composed mostly of a zirconium compound and is not hurt 
by exposure to the air. It should be heated up gradually as directed for the 
limes ( 162). Moving Picture World, June 13, 1914, p. 1539. 



IO2 



MAGIC LANTERN WITH THE LIME LIGHT [Cn. IV 



time smaller cylinders with the gas at a much higher pressure (100 
to 120 atmospheres) are employed (see also 156). In using the 
gas it is drawn off through a reducing valve by which it can be 
delivered at any pressure desired, and of course in any volume 
desired. 

One should never try to use the gas without drawing it through 
the reducing valve. The cylinders have special junctions for the 
reducing valve, so that it is easy to make the connections. 




FIG. 57. OXYGEN CYLINDER 
WITH COMPRESSED OXYGEN, 
THE PRESSURE GAUGES AND 
THE MIXED JET OR BURNER. 

(Catalogue of Schmidt and 

Haensch). 

B Tip of the nozzle of 
the mixed jet. 

K Holder for the lime. 
The end of the lime is used, 
not the side as in fig. 56, 59. 
G Handle of the stop-cock 
for hydrogen in the tube of 
the burner. 

S Stop-cock for oxygen. 
// The tube conveying 
hydrogen to the burner, steel 
cylinder not shown. 
O Tube conveying oxygen from the steel cylinder to the burner. 
/ The high pressure gauge giving the number of atmospheres under which 
the gas in the cylinder is compressed. 

M The low pressure gauge to show the pressure of the gas after it has 
passed the pressure reducing valve (St.). 

St The handle of the valve serving to open the pressure reducing apparatus. 
V The valve of the cylinder. This must be opened to allow the compressed 
gas to escape into the tube passing to the reducing valve and to the high pres- 
sure gauge. It must be closed after every exhibition. 



CH. IV] MAGIC LANTERN WITH THE LIME LIGHT 103 

In Great Britain and on the Continent oxygen cylinders are, by 
common usage, painted black, and the screw threads are right- 
handed. 

Hydrogen cylinders are painted red, and their screw threads 
are left-handed. 

In the United States of America this uniformity of color and dis- 
tinction of screw threads is not always found. 

155. Hydrogen in steel cylinders. Hydrogen gas is also 
compressed in steel cylinders, and forms an article of commerce. 
It must also be drawn off through a pressure reducing valve. 

Every precaution should be taken to avoid mixing the two gases 
in large quantities. Safety lies in mixing the gases only at the 
moment of exit from the two tubes of the blow-through jet or in 
the small mixing chamber of the mixed jet. 

156. Pressure gauges for gas cylinders. While a pressure 
reducing valve is a practical necessity, the pressure gauges are 
highly desirable. 

The one beyond the pressure reducing valve is a low pressure 
gauge and may indicate the pressure in millimeters of mercury or 
in centimeters of water (or, of course, in inches of water or mercury). 
This shows the pressure under which the gas is actually being used. 

The gauge next the cylinder registers the full pressure within. 
The figures on the dial usually represent atmospheres of pressure, 
one atmosphere being 760 mm. of mercury. The special purpose 
of this gauge is to enable one to determine the amount of gas in the 
cylinder at any given time, hence it is sometimes called a "capacity 
meter" or a "finimeter." 

If the pressure gauge does not indicate directly the atmospheric 
pressure, it may give the number of pounds per square inch or the 
number of kilograms per square centimeter. To change these to 
atmospheres one can use the approximate values: 15 Ibs. per 
square inch = i atmosphere; or i kilo per square centimeter = i 
atmosphere ( is6a). 

156a. The exact values are: 

One atmosphere equals 14.73 pounds per square inch. 

One atmosphere equals 1.033 kilograms per square centimeter. 



104 MAGIC LANTERN WITH THE LIME LIGHT [Cn. IV 

For example, suppose the pressure gauge indicates 1800 Ibs. per 
square inch, then it would be under a pressure of 1800 -r- 15 = 120 
atmospheres. 

If the pressure gauge should read 100 kilograms per square 
centimeter, then it would be under approximately 100 atmospheres 
pressure. 

From Boyle's law of the relation between the volume of a gas 
and the pressure to which it is subjected it is known that if one 
starts with a cylinder holding five liters of oxygen or hydrogen, or 
indeed of any other gas, under one atmosphere pressure, it will hold 
twice as much under two atmospheres, etc., so that a cylinder of 
five liters capacity at one atmosphere, will hold 500 liters at 100 
atmospheres pressure. Now to determine the amount of gas 
present in a given cylinder with the high pressure gauge one must 
know the capacity of the cylinder under the ordinary atmospheric 
pressure, and multiply this amount by the number of atmospheres 
of pressure indicated on the gauge. For example, if the capacity of 
the cylinder is i o liters at one atmosphere (often called no pressure) 
and the high pressure gauge indicates that the gas in the cylinder 
is under a pressure of 25 atmospheres, then the amount of gas is 
10 X 25 = 250 liters of gas; and so in like manner with any other 
pressure. For example, in England, the cylinders are filled under 
120 atmospheres pressure; this would give in the above case 
10 X 120 = 1200 liters to the full cylinder. 

On the Continent, the filling pressure is often 100 atmospheres 
and the cylinder of the same capacity would then contain 10 X 100 
= 1000 liters of the gas. 

The practical application of this knowledge is to determine in a 
given case whether there is sufficient of the gases present for the 
exhibition. Authors differ somewhat in estimating the amount 
of gas used per hour with the lime light lantern. A conservative 
estimate would be, for oxygen, 85 liters (about three cubic feet) 
and, for hydrogen, something over twice that volume, as, in prac- 
tice, there is an excess of hydrogen ( 161). 

157. Limes. The masses of unslaked lime (calcium oxid) 
used for the lime light are usuallv cylindrical in form. For some 



CH. IV] MAGIC LANTERN WITH THE LIME LIGHT 105 

burners they are placed on a pin or axle, and then must have a 
corresponding central hole. With other burners they are pressed 
into place between surrounding springs, somewhat as a lamp 
chimney is put on its burner (fig. 57, 59). 

The limes are sealed hermetically in glass tubes, or are packed in 
powdered unslaked lime, in air-tight tin cans, to prevent the access 
of moisture. 

If moisture reaches the limes they will slake and become powdery 
and useless for the light. To avoid any moisture reaching them 
they should not be removed from their protective covering until a 
few minutes before they are to be used. 

158. Lamp for the lime light. This consists of a burner or jet 
for conducting the two gases, oxygen and hydrogen, to a point 
where they can be mixed and burned ; and a device for holding the 
lime in a proper position, and raising, lowering, rotating and adjust- 
ing the lime with reference to the burner. 

There are two principal forms of burner or jet : 

(1) The blow-through jet. In this a stream of oxygen is blown 
into a flame of hydrogen on the principle of the gas or alcohol blow- 
pipe (fig. 58). 

(2) Mixed jet. In this form the two gases (oxygen and hydro- 
gen) meet and mix in a common chamber just before the nozzle 





H O H O HO 

FIG. 58. FORMS OF BLOW-THROUGH JETS (Lewis Wright). 

The form c shows best that the principle is that of a blowpipe. 

The form d approaches the mixed jet somewhat. 

With all of them the hydrogen, or hydrogen substitute (illuminating gas, 
ether or gasoline vapor) passes out from the supply through the tube marked 
H at the left. The oxygen is then blow r n through the flame from the tube at 
the right marked 0. Not so much light can be got with these jets as with the 
mixed jet, but for illuminating gas or ether vapor, etc., this form, especially 
a, b, c is safer in the hands of amateurs than the mixed jet. 



106 MANAGEMENT OF THE LIME LIGHT [CH. IV 

opens; then the mixed gases burn on emergence from the nozzle 
(fig. 59). This form of jet gives the greater amount of light but 
the two gases should be under considerable pressure. The tip of 
the nozzle (fig. 59 N) makes an angle of 40 or 45 degrees with the 
lime. This gives a source of light above the tip of the nozzle, and 
hence there is free passage for the light to the condenser. 

The blow-through jet is usually 10 to 15 mm. (y 2 inch) from the 
lime while for the mixed jet the nozzle is within about 3 mm. (y% 
inch) of the lime. 

MANAGEMENT OF THE LIME LIGHT 

159. Connecting the gases with the burner. This is accom- 
plished by means of rubber tubes of thick walls, and the ends of 
the tubes should be tied or wired to the supply pipes and to the 
burner ( i59a). 

It is a great advantage to have the two parts of conducting tubes 
of the burner of the same color as the gas tanks, viz., red for hydro- 
gen and black for oxygen, then there will be less liability to error 
in connecting the gas supply. 

It is only while using the gas that the cylinder valve (fig. 57 V) 
should be opened. And in opening it care should be taken to open 
slowly so that the sudden rush of the compressed gas may not injure 
the pressure gauges or the reducing valve. 

When through with the cylinder at any time the cylinder valve V 
should be closed. 

The pressure of the two gases should be about equal. This can 
be arranged by the pressure reducing valve. Set this to give the 
desired pressure, which ordinarily is equal to a column of water 
about 28 to 50 cm. high (n to 22 inches) or 2 to 4 cm. of mercury 
(^4 to 1^2 in. Hg) a pressure of .03 to .06 kilos per sq. cm. (.4 to .8 
Ibs. per sq. in.). 

160. Lighting the jet. Turn on the hydrogen slightly and 
light it with a match or a cerium-iron gas lighter, then continue to 



159a. Flexible metallic tubes. There is now available flexible metallic 
tubing with rubber connections at the ends to use in place of rubber tubes for 
conducting gases (fig. 60). 



CH. IV] MANAGEMENT OF THE LIME LIGHT 

N 



107 




FIG. 59. MIXED BURNER OR JET FOR THE LIME LIGHT. 
(From the Catalogue of Williams Brown & Earle). 

H H The metal tube of the burner conveying the hydrogen to the mixing 
chamber (M}. It should be painted red to correspond with the color of the 
hydrogen cylinder of compressed gas. 

The metal tube conveying oxygen to the mixing chamber (M). It 
should be painted black to correspond with the color of the oxygen cylinder of 
compressed gas. 

M The common chamber into which open the oxygen and hydrogen tubes. 
Here the gases mix before passing out through the nozzle (N). 

N The nozzle or outlet tube from the mixing chamber. It is at an inclina- 
tion of about 40 to 45 degrees with the vertically standing lime face; and 
when the burner is in action the nozzle and lime are about 3 mm. (J/gth in-) 
apart. 

L The support and springs for holding the lime. 

5 The milled heads of the pinions by which the lime is rotated or raised and 
lowered. The lime support slides back and forth on the supply tubes O and // 
so that the lime may be withdrawn from or made to approach the tip of the 
nozzle (N). 

open the stop-cock until the flair.e is from 8 to 15 cm. (3-6 in.) long. 
Then turn on the oxygen slowly until the flame just commences to 
hiss. After the lamp has been going some minutes the operator 
can slightly increase or decrease the oxygen until the most brilliant 
light is obtained. One must learn by experience. The flame will 
become very small as the oxygen is turned on, and this small, 
intensely hot flame heats a very small part of the lime, hence the 
source of light is very small, something like the crater of the posi- 
tive carbon in the direct current arc lamp. 

Caution. Always turn on the hydrogen first and light it before 
turning on the oxygen. 

Never turn on the oxygen first, and never until the hydrogen 
has been lighted, and then turn it on slowly. 

If both were turned on before lighting the hydrogen, there would 
be a greater or less explosion. This might not be very dangerous, 



io8 MANAGEMENT OF THE LIME LIGHT [Cn. IV 

but it has a dangerous sound; and the purpose of the exhibition is 
to instruct or entertain, not to scare the audience. To insure the 
correct use of the gases it is a good plan to have the stop-cock 
handles of the two gases so different that one can tell by feeling 
which one is being turned on. 




FIG. 60. FLEXIBLE METALLIC TUBING WITH RUBBER CONNECTORS AT 

THE ENDS. 

(Cut loaned by the Pennsylvania Metallic Tubing Co.). 

161. Regulating the flame. Theoretically the proportion of 
the two gases should be their combining quantities (H 2 O) ; but 
experience has shown that better results are gained when the 
hydrogen is in excess. When the oxygen is in exactly the combin- 
ing proportion there is liable to be a snap and the light goes out. 
If there is an excess of hydrogen this docs not happen. As stated 
above, the oxygen should be added until the flame just begins to 
hiss. 

162. Putting a lime in position. A fresh lime from the box 
should be put in position in the burner (fig. 59 L) before lighting 
the hydrogen, but the lime should at first be 3 to 5 cm. (i to 2 in.) 
distant from the tip of the nozzle (fig. 59 N), and it should be 
rotated, raised and lowered until it is warmed. If the full heat of 
the O-H flame were directed against one point of the cold lime for 
too long a time the lime would be liable to break. After it is well 



CH. IV] 



MANAGEMENT OF THE LIME LIGHT 



109 



warmed up the lime is not liable to break. Some operators warm 
the lime by means of the hydrogen flame only. When the lime is 
warm the oxygen is turned on slowly until the most brilliant light 
is obtained. 

163. Arranging the lime and the burner ; rotating the lime. 

After warming the lime for half a minute or so it should be grad- 
ually brought toward the nozzle until it is only about 5 mm. (% 
inch) distant. If now one watches the disc of light on the screen 
and slowly moves the lime slightly closer to and farther from the 
tip of the nozzle it is easy to tell when one gets the most light. It 
is to be remembered that the best light is not practically instan- 
taneous, as with the arc lamp, but is produced after the lime has 
been half a minute or so in one position. 

164. Changing the position of the lime. The intense heat of 
the oxy-hydrogen jet makes a little pit in the surface of the lime. 
In about two minutes this pit gets so deep that the light is greatly 




FIG. 61. GLOVES WITH ASBESTOS PATCHES ON THE THUMB, INDEX AND 
MIDDLE FINGERS FOR USE IN WORKING ABOUT THE HOT LIME- 
LIGHT LANTERN. 

Right hand, palm up: p, Pollex or thumb; i, Index or fore finger; m, 
Medius or middle finger; c, the index and medius used as pincers to grasp a 
hot lime or a hot carbon. 

Left hand, palm up: i, 2, j The first, second and third digits, but num- 
bered instead of named. 



no MANAGEMENT OF THE LIME LIGHT [Cn. IV 

lessened, and one must move the lime a little so that a new surface 
may be acted upon. 

In practically all the modern burners there is a screw mechanism 
for rotating the limes and for raising and lowering them (fig. 59 S). 
With a little experience one learns by the looks of the screen 
light when to turn the lime. If the limes must be handled, use 
tongs or asbestos-patch gloves (fig. 61). 

165. Turning out the light. Always turn off the oxygen first, 
then the hydrogen. Never turn off the hydrogen until after the 
oxygen is turned off. 

Perhaps it will help to remember the order by keeping in mind 
that (i) the Hydrogen is the first to come and the last to go. (2) 
And the Oxygen, like the best in human nature, is last to come and 
first to go. 

MANAGEMENT OF THE LIME LIGHT MAGIC LANTERN FOR AN 
EXHIBITION OR DEMONSTRATION 

166. Preparation for an Exhibition. Before the exhibition 
the operator should see that everything is in perfect order and 
readiness. The gas cylinders should be connected with the burner, 
and a perfect, fresh lime should be in position in the burner. The 
box of limes should also be at hand in case anything goes wrong 
with the one in the burner. 

167. To start the light. It takes much longer than for the 
arc lamp. It is usually about half a minute before the brightest 
light possible is produced, and one must not forget the precaution 
to warm the lime before subjecting one spot to the full power of the 
O-H jet. 

Light up as directed above ( 160). 

If there is a snap and the light goes out, turn off the oxygen, and 
relight the hydrogen. Turn on the oxygen slowly until the best 
light is obtained ( 160). 

168. To put out the light. Turn off the oxygen first, then the 
hydrogen ( 165). 



CH. IV] 



MANAGEMENT OF THE LIME LIGHT 



in 



169. Shield for cutting off the light from the screen. As it 

takes considerable time to start the lamp after it has been put out 
it is not so easy to use the lime light intermittently as the arc lamp, 
hence in a lecture or demonstration in which the lantern slides are 
to be shown at several different times, it is best to leave the lamp 
burning all the time. But the screen should not be lighted all the 
time, and to avoid this the objective shield (fig. 62) may be used. 




FIG. 62. SHIELD FOR THE OBJECTIVE IN INTERMITTENT PROJECTION, 
WHEN SLOW-LIGHTING RADIANTS ARE USED. 

S l Shield up to allow the light to pass from the objective to the screen. 

5 1 Shield down to cut the light off from the screen. This shield is especially 
desirable when slides are to be shown at intervals, as in a demonstration lecture 
with the lime light, a Nernst light, a kerosene light, or an alcohol light ( 169). 

Sometimes also to avoid using so much gas and burning out the 
lime too quickly there are regulating valves, by which only a small 
amount of the two gases is allowed to pass, without changing the 
relative proportions. When these valves are opened again the full 
amount needed and in the original proportions is allowed to flow 
again. Even in this case there should be a shield before the objec- 
tive to avoid lighting the screen. 

170. Proper lighting for the screen. The light on the screen 
should be uniformly brilliant. This can be attained by following 
the directions for centering and getting the proper distance of the 
lamp from the condenser exactly as with the direct current arc 
lamp ( 51-57). 

If there arc shadows on the screen make the proper change in the 
position of the lamp, etc., as indicated in fig. 27-30, 83-93. 



H2 LIME LIGHT WITH OXYGEN AND GAS [Cn. IV 

If everything has been put in perfect order before the exhibition 
the changes required during the exhibition should be very slight. 

171. Arrangement of lantern slides, their insertion and 
focusing. Follow the directions in Ch. I, 21-23; 35~4 I - 

172. Lighting the room. As the lime light gives only about 
Vs to VG as much light as the arc lamp the room must be darker 
if the same brilliant contrast is desired. One can determine by a 
little experiment with the set of slides to be exhibited at any time 
how dark to have the room. The more transparent the lantern 
slides, the lighter can the room be. Many lantern slides are 
altogether too opaque, and require a dark room, no matter what 
light is used in the lantern. 

173. Avoidance of intervals of total darkness in the room. 

This can be accomplished by leaving the lantern on all the time and 
by using the objective shield (fig. 62). If that device is not used, 
then the operator should not turn out the lime light until the room 
lights are turned on. And whenever the lantern is to be used, the 
lecturer must give two or three minutes warning to the operator 
before turning off the room lights. 

THE LIME LIGHT WITH OXYGEN AND ILLUMINATING GAS 

174. Frequently the lime light is produced with illuminating 
gas drawn from the house supply, and with oxygen gas in a steel 
cylinder ( 154). 

If illuminating gas is used instead of hydrogen it is to be remem- 
bered that the pressure as drawn from the house supply is very 
slight, i. e., about equal to a column of water from 5 to 12 cm. 
high (2 to 5 in.) or only about Vr, the pressure of the hydrogen 
and oxygen when these gases are drawn from steel cylinders ( 1 54). 

The oxygen is used at a much higher pressure than the house gas, 
and many operators use for this combination the "blow-through 
jet" (fig. 58). Mixed jets arc also constructed for this combina- 
tion, but the "blow-through" is considered safer. The user of this 
form of apparatus would do well to get the combination found best 
by the manufacturers of his apparatus. 



CH. IV] OXYGEN GENERATOR AND ETHER SATURATOR 113 



175. For lighting the lamp. Whatever form of burner is used 
turn on the illuminating gas first and light it; then turn on the 
oxygen until the flame is made much smaller, as with hydrogen. 
For warming and arranging the lime and its distance from the 
nozzle of the jet see 158, 162-164. 

176. Putting out the lamp. Turn off the oxygen first, then 
the illuminating gas. 

Remember that oxygen is always on last, and off first. 

LIME LIGHT WITH OXYGEN GENERATOR AND ETHER SATURATOR 

177. Oxygen generator. There has recently been perfected 
a method of preparing sodium peroxide so that it gives off oxygen 
gas when water is added, somewhat as 
calcium carbide gives off acetylene gas 
when put in water. This substance gives 
about 300 times its volume of oxygen, 
and serves very well for an oxygen sup- 
ply when used in a proper generator. 

178. Hydrogen substitute. The 

substitute for hydrogen with this outfit 
is sulfuric ether or gasoline. But ether 
and gasoline should never be mixed. 

179. Use of the apparatus. There 
must be a burner and lime holder as for 
the oxy-hydrogen lime light. The sodi- 
um peroxide (Oxone, oxodium, oxylithe 
are trade names) is put into the generator 
and the oxygen gas conducted over to the 
ether saturator. In the saturator, the 
stream of oxygen from the generator is 
divided, one stream of the oxygen going 

directly to the burner through one tube, F]G 63. PORTABLE OXYGEN 
and another part going through the ether 
chamber of the saturator and becoming 
loaded with ether vapor. This oxygcn- 




GENERATOR AND ETHER 
SATURATOR. 



H4 TROUBLES WITH THE LIME LIGHT [Cn. IV 

ether vapor is inflammable and takes the place of hydrogen or 
coal gas. The pure oxygen mixed with it just before it emerges 
from the burner gives the necessary intensity to the flame. 

In using this outfit it is necessary to follow very precisely the 
directions of the manufacturers to avoid accidents. In particular, 
one must be sure to turn on the oxygen-ether first and light it; 
then turn on the pure oxygen until the light is best. In turning 
the light out: Turn off the oxygen first, then after a moment, 
turn off the oxygen-ether supply. 

The oxygen produced from one charge of 3^ pounds of the 
sodium peroxide (oxone) gives about 6.6 cubic feet of oxygen gas, 
enough to last from two to three hours for the magic lantern. One 
filling of the ether saturator requires about one pound of sul- 
furic ether and will supply the ether vapor for the charge of 
oxone. It is said by the manufacturers that if used economic- 
ally the single charge of oxone and ether will supply a double lan- 
tern for an entertainment lasting an hour or an hour and a half. 

TROUBLES WITH THE LIME LIGHT 

180. Snapping out of the light. This is usually due to an 
excess of oxygen. The oxygen should always be less than the 
hydrogen or any of its substitutes, i. e., illuminating gas, ether or 
gasoline vapor, acetylene gas. To invert the statement, the 
hydrogen or its substitutes, i. e., the inflammable gas or vapor 
should be in excess of the actual combining proportions. If the 
lime is too close to the burner tip the light will snap out. 

In case the light snaps out, at once turn off the oxygen. Light 
the hydrogen and slowly turn on the oxygen again until a satis- 
factory flame is obtained. Be sure the lime is not too close to the 
burner tip. 

181. Going out of the light. This may be due (i) to a lack of 
one or of both the gases used, that is, the supply may be exhausted. 
Look at the capacity meter. 

(2) Some of the valves may be clogged. 

(3) A rubber tube may have split or come off at the connection. 



CH. IV] TROUBLES WITH THE LIME LIGHT 115 

(4) A lime may have broken so that there is nothing for the 
hot flame to make incandescent. 

Remedy. Turn off the gases the first thing; oxygen first then 
the hydrogen or other gas. One can then investigate each of the 
possible causes for the going out of the lamp. The broken lime 
and the split or separated rubber tube can be most easily detected 
and corrected, and consequently should be looked for first. 

182. Irregular light or shadows on the screen. The fault 

may lie in any of the following, to name which is to suggest a 
remedy : 

(1) The lime may be too deeply pitted where the flame strikes 
it. Change the position of the lime ( 164). 

(2) The lime may be in bad position, too high or too low, too 
far from or too close to the burner tip. 

(3) The incandescent spot may not be centered on the axis, i. e., 
be too high or too low; too far to the right or to the left with the 
resulting shadows as with the crater of the arc lamp (fig. 27-30). 

(4) The light may be too close to or too far from the condenser. 

(5) The nozzle of the burner may be in the way and cast a 
shadow. If so, it must be lowered or the distance from the lime 
or the angle changed (see also 82-91). 

183. Roaring or hissing of the burner. A slight hissing sound 
is usually heard when the right amount of oxygen is being used. 
But when the roaring becomes annoying its cause must be found 
and remedied. It may be due to: (i) The inside of the nozzle 
tube may be rough. 

(2) The lime may not be the right distance from the tip of the 
nozzle. 

(3) The pitting of the lime may be too great. 

(4) There may be too great a supply of the gases for the bore 
of the nozzle. 

184. Cracking of the lime. This is usually due to a sudden 
heating of the lime. If it is warmed gradually by rotating it at 
first at some distance and then closer to the flame the breaking is 
usually avoided. If broken, the lime should be removed from the 



Ii6 TROUBLES WITH THE LIME LIGHT [Cn. IV 

holder and a new one put in place. This should then be grad- 
ually warmed ( 162). 

SPECIAL PRECAUTIONS IN USING THE LIME LIGHT 

185. Remember that hydrogen and all the substitutes used 
for it, illuminating gas, ether and gasoline, are very inflammable. 

Oxygen with hydrogen and also with the other substances forms 
an explosive compound. Hence, the greatest care must be taken 
to avoid mixing these gases except in the mixer of the burner 
(fig. 59). Hence also in filling any part of the apparatus and in 
working about it there should be no open flames or glowing parts 
to ignite any accidentally escaping hydrogen, gasoline, ether, etc. 
Fill the apparatus by daylight, or use an electric light or an elec- 
tric flash-light if the work must be done in a dark place. In this 
way no chance for igniting the gases will occur. Naturally one 
should not smoke when filling the apparatus. 

It is economical to buy the best apparatus throughout. The 
makers adapt the burners and all other parts to give the best 
results in the safest manner, therefore, unless one is an expert in 
such matters it is safer to take the outfit assembled and recom- 
mended by some reliable manufacturer. 

The makers send out with their apparatus very precise directions 
for using it with safety, and it is the height of wisdom to follow 
their directions faithfully. 



CH. IV] 



DO AND DO NOT WITH THE LIME LIGHT 



117 



186. Summary of Chapter IV: 



Do 

1. Use gas cylinders which 
are plainly marked Oxygen 
and Hydrogen, and have right- 
handed screws for the oxygen 
and left-handed screws for the 
hydrogen ( 154). Be sure that 
there is plenty of gas in each 

( 156)- 

2 . Connect the cylinders with 
the burner by means of rubber 
or metallic tubing, colored to 
correspond with the cylinders 
(OorH) ( 154, 159, i5 9 a). 

3. In starting the burner, 
turn on the hydrogen or its 
substitute first and light it, 
then turn on the oxygen slowly 
( 160). 

4. Heat up the lime slowly by 
having it at some distance from 
the flame ( 162). 

5. Turn the lime occasionally 
so that the pit will not get too 
deep ( 164). 

6. In putting out the lamp, 
turn off the oxygen first, then 
the hydrogen after a moment. 

7. If the light snaps out, turn 
off the oxygen then the hydro- 
gen. Turn on the hydrogen, 
light it and then turn on the 
oxygen slowly as in (3). 



Do NOT 

i. Do not use gas cylinders 
which are not plainly marked. 
Do not start an exhibition 
unless there is plenty of gas. 



2. Do not be careless in con- 
necting the cylinders with the 
gas burner. 



3 . Do not turn on the oxygen 
first. Oxygen is last on, first off. 



4. Do not turn the full heat 
of the O-H flame against a cold 
lime which is close up to it. 

5. Do not let the lime stay 
too long in one position. Ro- 
tate it occasionally. 

6. Do not turn off the hydro- 
gen first, but turn off the 
oxygen first. Oxygen is on last, 
off first. 

7. Do not leave the gases 
turned on if the light snaps out. 
Oxygen off first, then Hydrogen. 



n8 



DO AND DO NOT WITH THE LIME LIGHT [Cn. IV 



8. After the exhibition is over 
remove the lime or it will 
slake in the holder. 



8. Do not leave the lime in 
the holder to slake after the 
lecture. 



9. Conduct the exhibition 
exactly as with an electric 
lantern (Ch. I, 21-40). 



10. As the hydrogen or its 
substitute is inflammable, and 
the oxygen is a perfect supporter 
of combustion, follow the direc- 
tions given by the manufac- 
turers of a special apparatus 
intelligently and exactly. 



9. Do not spare any pains in 
conducting an exhibition with 
the lime-light magic lantern. 
More care and skill are neces- 
sary than with the electric light 
lantern. 

10. Do not take any chances 
when dealing with the oxy- 
hydrogen lantern. Do things 
in the right order, and do not 
neglect the directions of the 
manufacturers. 



CHAPTER V 

MAGIC LANTERN WITH PETROLEUM LAMP; VERTICAL 

AND REFLEX MANTLE GAS LAMPS; ACETYLENE 

LAMP; ALCOHOL LAMP WITH MANTLE 

190. Apparatus and Material for Chapter V: 

Suitable projection room with screen ; Magic lantern with lamp 
and chimney for petroleum (fig. 65-67); High grade petroleum for 
burning in the lamp; Gas burners for vertical and reflex mantles 
(fig. 68-69); Illuminating gas supply; Acetylene burner and 
reflector, (fig. 70) ; Acetylene gas supply (house supply, prestolite 
tank of compressed acetylene in acetone or an acetylene generator) ; 
Special alcohol lamp with mantle (fig. 72-73); Strong alcohol 
(95%) ethyl, methyl or denatured. The magic lantern for all but 
the oil lamp must have a lamp-house into which the burner can be 
placed. There must be lantern slides, screw drivers, pliers and 
matches or safety lighters ( 160), for all of them. 

191. Historical development and references to literature. 

For the history see the Appendix, and for general works of reference 
see the list of books in the first chapter (2). 

The directions sent out by the manufacturers of these light 
sources should be studied carefully and followed exactly unless one 
has technical knowledge on the subject. 

OIL AND GAS LAMPS 

192. Early sources of light. For a long time after the inven- 
tion of projection apparatus there were but two sources of light 
known : 

(1) The sun, which has ever remained the most brilliant source 
of light available, and 

(2) Some form of torch, candle, or oil lamp. 

The first oil lamps burned animal or vegetable oil and had no 
lamp chimney. 

After the discovery and proper refinement of petroleum, that 
became and has remained the oil most used for illumination. 

If one reads the early works on projection it seems astonishing 
that the workers of those times were able to produce screen images 

119 



120 



MAGIC LANTERN WITH OIL AND GAS LAMPS [Cn. V 



which showed general form and details with anything like satisfac- 
tion to large audiences. But screens as large as four meters square 
(12 ft. sq.) were used with the petroleum light. 

When the feeble lights discussed in this chapter are compared 
with the powerful electric arc light giving from 1,000 to 5,000 candle- 
power it would seem that the results of earlier times must have been 
very unsatisfactory. 

But the older lanternists gave very successful exhibitions. They 
did this by observing with scrupulous care the requirements for 
projection with their appliances. 



Condenser 




FIG. 64. MAGIC LANTERN WITH LARGE LIGHT SOURCE. 

Lamp Illuminating gas lamp with Welsbach mantle. 
Condenser Triple-lens condenser without water-cell. 
S Lantern slide. 

Objective Projection objective with inverted image of the luminous mantle 
between the lenses. 

Screen Image The image of the lantern slide on the white screen. 

193. Requirements for projection with a feeble light: 

(A) The lantern slides must be very transparent ; and the old, 
hand-painted slides were very transparent. 

(B) The room must be very dark. There must be no stray 
light from the windows or from the apparatus ; the only light must 
be that issuing from the lantern objective and reflected from the 
screen . 

(C) The management of the lantern must be the best possible, 
so that all the available light may be utilized for producing the 
screen image. 

(D) The projection objective must be of large aperture so that 
as much as possible of the light issuing from the large source (lamp 



CH. V] MAGIC LANTERN WITH PETROLEUM LAMP 121 

flame or incandescent mantle) , may be utilized in making the screen 
image. This is of fundamental importance (fig. 64, 90). 

(E) Use of twilight vision. It is astonishing how dim a picture 
can be clearly seen after one's twilight vision has become fully 
established. According to careful investigations the sensitiveness 
of the eye may be increased from 35 to 2500 times by the adapta- 
tion to dim light ( 281). 

The old lanternists used to advise that the exhibition should not 
begin until the audience had been in the darkened room for half an 
hour "to get," as they said, "the sunlight out of their eyes." We 
would say to "get the twilight vision well established." 

194. Time required for lighting up. The gas light and the 
acetylene light are quickly established, but the petroleum and 
the alcohol lights require several minutes to get up the best illumi- 
nation. These two should then burn during the entire time of an 
exhibition. If the lecturer cannot arrange to have all the slides 
continuously, but must have them at intervals during the lecture, 
the operator should make use of an objective shield (fig. 14, 62), 
and leave the lights on all the time. 

195. Rehearsals. As these lights are more difficult to man- 
age and the results are less satisfactory than with the more power- 
ful radiants, so much the more should the operator rehearse before 
the lecture and make sure that everything is in as nearly perfect 
order as human skill can make it. 

THE MAGIC LANTERN WITH A PETROLEUM LAMP 

196. The petroleum lamps now used as radiants for projec- 
tion have two, three or four wicks. The wicks are wide (about five 
cm., two in.) and are placed edgewise to the condenser. If more 
than two wicks are used the two outer ones are inclined inward 
(%. 66). 

Sometimes instead of being ranked side by side, the different 
wicks are arranged like the lines forming the letter W, but there is 
no advantage in this. 



MAGIC LANTERN WITH PETROLEUM LAMP [Cn. V 




FIG. 65. MARCY'S MAGIC LANTERN OR "SCIOPTICON" WITH A 
MULTIPLE-WICK, PETROLEUM LAMP. 

(From Dolbear's Art of Projecting). 

a-b. c-d The lenses of the projection objective. 

p-q The condenser lenses. 

Z S The oil reservoir of the lamp. 

E The flames of the lamp with their edges toward the condenser. 

G-G Two glass plates at opposite ends of the lamp-house to allow the light 
to pass to the condenser, and so that the reflector II can return the backward 
extending light. 

C I J The chimney and ventilator of the lamp-house. 

W W At the right, the milled heads for turning the lamp-wicks up or down. 

There is a common reservoir and a common chimney, but each 
wick has a separate burner and a separate mechanism for raising 
and lowering the wick. 



197. Chimney and reflector. There is a common chimney. 
This is usually of metal with a window on opposite sides, and with 
either a telescoping extension or a segment which can be put on top 



CH. V] MAGIC LANTERN WITH PETROLEUM LAMP 123 

for getting the best draught when the lamp is turned up full 
height. 

The reflector is a concave mirror placed with its center of curva- 
ture coinciding with the flame. This serves to reflect the backward 
extending light to a focus on the flame again, and from thence it 
passes onward to the condenser with the rays passing directly from 
the flame to the condenser. 




FIG. 66. MULTIPLE-WICK, PETROLEUM LAMP FOR THE MAGIC LANTERN. 
(From the Catalogue of the Mclntosh Battery and Optical Company, 7889). 

This figure shows that there is a single oil reservoir but four separate wicks, 
each with a mechanism for turning the wick up or down. It also shows 
clearly the inclination toward each other of the separate wick holders, and 
finally that the lamp has a single chimney. 

The openings in the metal chimney for the reflector and the con- 
denser must be covered with glass or with clear mica or the lamp 
will smoke. 

198. Management of the lamp. Before an exhibition the 
reservoir is filled nearly full with good petroleum (kerosene oil). 
The wicks must be carefully trimmed until the flame burns without 
tails. One must be careful in preparing the lamp not to get any 
oil on the outside, for when the lamp gets hot this oil is sure to smell 
badly. 



124 



MAGIC LANTERN WITH PETROLEUM LAMP [Cn. V 



Light the wicks and turn them up moderately and allow them to 
burn for five or ten minutes before the exhibition. This is to get 
the apparatus warmed up. One cannot get the best light from a 
petroleum lamp instantly, but only after it has become warm. 
Finally turn up each wick as high as possible without having it 
smoke. The central wicks can usually be turned higher than the 
marginal ones. When the wicks are at their full height the chim- 
ney, if adjustable, must also be at its full height to give the best 
draught. 

After the exhibition is over the lamp-wicks are turned down, the 
small flames blown out, and then the unused oil poured into a con- 
tainer, the wicks taken out and carefully dried between blotting 




FIG. 67. NEWTON'S FOUR-WICKED, PETROLEUM LAMP KOR THE MAGIC 

LANTERN. 

(From Catalogue No. 4 of Newton r Co.}. 

The chimney is in two segments. For the maximum light after the lamp is 
warmed up, the top segment is added. 



CH. V] MAGIC LANTERN WITH MANTLE GAS LAMP 125 

papers. If the lamp is kept perfectly clean, and no oil is allowed to 
remain on the outside, the disagreeable smell of partly oxidized oil 
will be avoided. 

199. Amount of oil used. It takes about half a liter (one 
pint) of kerosene per hour for the best lamps. 

200. Candle-power and size of screen. The candle-power of 
the best petroleum lamps is not much above 100. While the older 
lanternists used large screens (4 meters, 12 ft. square) it is better 
to use, with this light, screens of small size, 2 to 3 meters square 
(6-9 ft.), and to keep in mind the requirements for good images 
with these feeble lights ( 193). 

201. Relative position of lamp and condenser. In general, 
the middle of the flame should be in the axis of the condenser and 
it should be at about the principal focal distance of the first ele- 
ment of the condenser from it (fig. 64). One must get the best 
possible position at any one time by experiment, i. e., by moving 
the light a little closer or farther away than the focus of the con- 
denser. For the two-lens condenser the lamp must be closer than 
for the three-lens condenser ( 17, 55). 

202. The management of an exhibition is as described in 
Chapter I, 21-41, and above, 193-194. 

MAGIC LANTERN WITH A MANTLE GAS LAMP 

203. Gas and gas lamps. The illuminating gas may be 
drawn from the house lighting supply. 

The lamps are of two kinds, the vertical and the inverted or 
reflex form (fig. 68-69). The burner is of the Bunsen type. It 
heats the mantle to incandescence. While there is a very brilliant 
light and a great deal of it, the source is very large, and cannot be 
utilized so completely as the small source of the electric arc lamp 
(see fig. i, 64). 

204. Position of the incandescent mantle. As this is the 
source of illumination, the middle of the face next the condenser 
should be on the horizontal axis (fig. 64). 



126 



MAGIC LANTERN WITH MANTLE GAS LAMP [CH. V 




FIG. 68. UPRIGHT GAS BURNER WITH WELSBACH MANTLE AND 
CONCAVE REFLECTOR FOR THE MAGIC LANTERN. 

(From Max Kohl, A. G., Price List No. 50, Vol. I). 

The distance from the condenser giving the best light must be 
determined by experiment, as with 
other extended sources. But, in gen- 
eral, it will be found to be at about 
the principal focal distance from the 
first element of the condenser, as 
with the arc lamp, but closer for the 
two-lens than for the three-lens con- 
denser (55). 

205. Reflector. As with the pe- 
troleum light, a concave reflector is 
sometimes used behind the mantle to 
reflect back to the mantle and thence 
to the condenser the light which passes 
backward from the mantle. This is 
not always used, but it would increase 
the light somewhat ( 210). 

206. Connecting the gas supply 
with the lamp. Use for this a perfect 
rubber tube or one of the flexible me- 
tallic tubes (fig. 60), and secure the 




FIG. 69. INVERTED GAS BIR- 
NER WITH WELSBACH MAN- 
TLE FOR THE MAGIC 
LANTERN. 

(From Schmidt und llamch'a Cata- 
logue, A'o. IV, Projektions Apparte, 



CH. V] MAGIC LANTERN WITH ACETYLENE LAMP 127 

ends to their connections by tying a string tightly around 
them, if rubber tubes are used. If the supply is at a con- 
siderable distance there should be a stop-cock at the lamp to 
regulate the amount of gas, and to turn it off completely if desired. 
At the end of the exhibition the gas must be turned off at the source 
of supply. 

207. The management of the exhibition is simple, and should 
follow th3 general lines laid down in Chapter I ( 21-41). It is 
not wise to try to use a screen more than two to three meters square 
(6-9 ft.), and one must keep in mind the requirements for feeble 
lights ( 193). 

THE MAGIC LANTERN WITH AN ACETYLENE LAMP 

208. Source of acetylene. This may be from a house supply, 
a special generator, or from a tank or cylinder of acetylene dis- 
solved in acetone under pressure (prestolite tank). 

209. Acetylene lamp. The burners now used are in pairs. 
Two jets set at an angle give a fused, flat flame. For the magic 
lantern the lamp has from one to four of these twin burners in 
a line. Behind the burner is a concave reflector returning the 
backward reflected light to the burner and from thence on to the 
condenser, so that as much of the light as possible is utilized for the 

screen image (fig. 70). 

210. Position of the concave mir- 
ror. If a concave mirror is used to save 
the light extending away from the screen, 
its center of curvature should coincide 
with the flame of a single burner, or its 
center should be at the middle flame, if 
there are several burners in a row. 
FIG. 70. DOUBLE-JET The acetylene flame is very transpar- 
R^Ko R L To' R "HE it, so that a mirror behind the burner 
MAGIC LANTERN. will increase the light nearly the theo- 

retical amount (75%)- while with nearly 




128 



MAGIC LANTERN WITH ACETYLENE LAMP 



[CH. V 



opaque sources, such as the incandescent mantle light or the 
petroleum flame, a mirror placed behind the light does not in- 
crease the brilliancy so much. 

211. Position of the acetylene lamp. This should be so that 
the middle point of the flame is on the axis (fig. 64) and it should 
be at a distance from the condenser of about the principal focal 
length of the first element of the condenser and the middle flame 
of the burner. For the best position in practice one must experi- 
ment while looking at the screen image or disc of light, and arrange 
the lamp to give the best effect ( 17, 55). 




B 



FIG. 71. UPPER AND LOWER ENDS OF A PRESTOLITE TANK USED \VITH 

THE MAGIC LANTERN. 
FIG. 7iA. UPPER END OF THE PRESTOLITE TANK. 

V Outlet valve. It is opened and closed by a special wrench. 

Connector The metal connector for joining the gas supply and the acetylene 
burner. 

Rt Rubber or flexible metal tube extending from the connector to the 
burner. 

N Nut for holding the conical part of the connector in gas-tight union with 
the hollow cone of the tank-valve. This valve must be set gas-tight before 
opening the outlet valve (V). 

FIG. 71 B. LOWER END OF THE PRESTOLITE TANK SHOWING THE 

PRESSURE GAUGE. 

P G Pressure gauge indicating the pressure of the gas within the tank. 
The pressure is given in atmospheres or in pounds per square inch or in both. 



CH. V] MAGIC LANTERN WITH ACETYLENE LAMP 129 

212. Connecting the burner to the gas supply. For this a 
heavy and perfect rubber tube or a flexible metallic tube (fig. 60) 
should be used and the connections with the supply and with the 
burner should be tied unless special fittings are present. 

As with illuminating gas, the best light is obtained when the 
correct amount of gas is delivered at the tip of the burner. If too 
much gas is flowing the jets will blow, and if too little, there will 
not be light enough. 

If a tank of compressed acetylene in acetone is used (fig. 7 1 A) , 
the adjustments must be made at the valve on the cylinder. If one 
turned this on full head and tried to regulate by the stop-cock at 
the burner the pressure accumulating in the rubber tube would 
probably blow the tube from its connections or burst it ( 2 1 2 a) . 



212a. Prestolite tanks supplying acetylene for the Magic Lantern. A 

steel cylinder is packed with asbestos and this is saturated with acetone. 
Acetylene gas is then pumped into the cylinder and is dissolved by the acetone. 

The tanks are charged under a pressure of approximately 15 atmospheres at 
i8J-^ degrees centigrade (65 F.) this is 15.82 kilos per square centimeter or 225 
Ibs. to the square inch. 

The tanks are of various sizes, and their holding capacities, under 15 atmos- 
pheres pressure, are as follows: 

"A" contains 70 cubic feet of gas, (1982 liters), cost $25.00 

"B" contains 40 cubic feet of gas, (1132.6 liters), cost $18.00 

"E" contains 30 cubic feet of gas, (849.5 liters), cost $15.00 

Motor-cycle tank contains 10 cubic feet of gas, (283 liters), cost $ 8.00 

The burner for a magic lantern requires from one to two cubic feet of acety- 
lene gas per hour. The motor-cycle tank full of gas will then supply light, for 
from five to ten hours. It costs less than $1.00 to have the tank recharged, 
hence, the cost of gas per hour is from 10 to 20 cents. 

It is of importance to know at any given time whether there is gas enough 
to last for an exhibition or for a number of exhibitions. As shown with the 
lime light the cylinders are supplied with a gauge showing the pressure of the 
gas within the cylinder, and from Boyle's law that the amount of a gas in a 
given space depends on the pressure, it is easy to determine at any time the 
amount of gas available. It is only necessary to know the capacity of the 
cylinder under ordinary atmospheric pressure and to multiply that volume by 
the number of atmospheres indicated on the pressure gauge (see also 156). 

For example, the gauge of a motor-cycle tank (fig. 71 B), shows that the 
pressure is 12 atmospheres, how many cubic feet of acetylene gas arc avail- 
able? 

As the tank under 15 atmospheres holds 10 cubic feet of gas its capacity at 
atmospheric pressure must be 10 -=- 15 = % of a cubic foot. If it holds % of 
a cubic foot under one atmosphere, under 12 atmospheres pressure it will hold 
% multiplied by 12 = 8 cubic feet. 

The tank will then supply gas for four or for eight hours of continuous light 
depending upon the capacity of the burner. 



130 



LANTERN WITH ALCOHOL LAMP AND MANTLE [Cn. V 



213. The management of an exhibition is as for the direct 
current arc lamp, keeping in mind the general statements in this 
chapter (Ch. I, 21-40; 193). 

THE MAGIC LANTERN WITH ALCOHOL LAMP AND MANTLE 

214. An alcohol flame burning in the air, is very hot. This 
has been taken advantage of to heat a mantle to incandescence in 
the same way that illuminating gas with a Bunsen burner heats a 
mantle to incandescence. 




FIG. 72. MAGIC LANTERN WITH THE ALCO-RADIAXT. 
(Cut loaned by Williams, Brown & Earle}. 
For the details see fig. 32 and 73. 

For the best results the alcohol is vaporized, and the vapor burn- 
ing in a special burner gives the Bunsen flame necessary to heat the 
mantle. 

The light is as intense or more intense than gas light with a 
mantle. 

215. Alcohol supply and burner. There must be a reservoir 

for alcohol (95% ethyl, methyl, or denatured). This is connected 
with the burner by means of a metal tube with a stop-cock. In 
use the reservoir is filled over half full, but must always have an 
air space above. Connected with this air space is a force-pump 
by which the alcohol is put under pressure. 



CH. V] LANTERN WITH ALCOHOL LAMP AND MANTLE 131 

216. Lighting the lamp. (i) Place the lamp in a metal tray; 
put a mantle in position over the burner, and burn it off as for a new 
gas mantle. 

(2) Place the heater or torch in position under the burner 
(fig. 73 L) . Wet the torch well with strong alcohol, using a pipette. 
Sometimes the torch is saturated with alcohol by pouring the 
alcohol upon it from a bottle before it is put in place under the 
burner. This is usually wasteful, as some alcohol is almost sure 
to be spilled. 




FIG. 73. ALCO-RADIANT, SHOWING THE PARTS. 
(Cut loaned by Williams, Brown & Earle). 

BM The mantle. 

BS The gas burner for the volatilized alcohol. 

L H The heater to start the volatilization. 

S The handle for opening and closing the air valve of the burner. 

R Valve for turning on and off the alcohol supply. 

W The tank holding the alcohol supply. 



Connection for the pressure tube. 

Rubber bull) for forcing air into the alcohol reservoir. 



The round 



object in the course of the rubber tube is an air reservoir to make the pressure 
steady. 



132 LANTERN WITH ALCOHOL LAMP AND MANTLE ICn. V 

(3) When the torch is in place and wet with alcohol, open the 
stop-cock from the supply tank (fig. 73 R), and then light the 
torch. The alcohol flame will heat the burner and stand-pipe, and 
the alcohol in the stand-pipe will be vaporized and pass over 
through the small pipe to the burner where it will catch fire and 
burn. Open the air intake partly. In using the lamp this air 
intake must be regulated as for a Bunsen burner, the more pressure 
the more the valve must be opened. 

Soon the mantle should begin to glow brightly from the burning 
vapor in the burner. When this occurs commence to put pressure 
on the alcohol tank (fig. 73 W). This is done by connecting the 
pressure apparatus by means of the rubber tube to the alcohol 
tank, at T, (fig. 73), and squeezing the bulb. 

In case the first burning off of the torch does not start the lamp 
one must burn it off again, but do not add the alcohol until the 
torch or heater is out, and then use a pipette. Relight the heater 
and it will almost surely start the lamp. 

Do not connect the pressure apparatus until the mantle com- 
mences to glow. If pressure were on the alcohol tank at first the 
liquid alcohol would be forced over from the stand-pipe into the 
burner and would run down on the torch and upon the table. 
Remember that alcohol is very inflammable and also very unman- 
ageable \vhen it is on fire, so be exceedingly careful. 

(4) As soon as the mantle begins to glow brilliantly consider- 
able pressure can be put on the alcohol tank. The greater the 
pressure the wider must the air-intake at the burner be opened and 
the more brilliant will be the light; but as the pressure increases 
the lamp roars more loudly until, when the pressure is considerable, 
it roars like a young blast furnace. By watching the results one 
can avoid the excessive noise, and still get a brilliant light. 

217. Management of the exhibition. This is in general like 
any other magic lantern, but as the light depends largely on the 
pressure regulation, one must be careful to keep up the proper 
amount of pressure during the entire time. Do not expect too 
much of this light. It gives fairly good lantern-slide images for a 
screen from two to three meters (six to nine ft.) square. As the 



CH. V] TROUBLES IN CHAPTER V 133 

source is large, one needs a good projection objective of large aper- 
ture (see 855). 

218. Putting out the lamp. As this lamp is difficult to light 
it should be kept burning during the entire exhibition. One can 
shut the light from the screen by the objective shield (fig. 62). 

At the close of the exhibition, take the lamp from the lamp-house, 
remove the rubber tube from the pressure apparatus to the tank 
to relieve all pressure on the alcohol. Close the supply valve so 
that no more alcohol can pass over to the stand-pipe. Close the 
air-intake of the burner. Use a sponge well wet with water and 
apply it to the burner as near the mantle as possible without touch- 
ing the mantle. The sponge will naturally rest against the small 
conducting pipe and the stand-pipe in this operation. This cools 
the burner and the stand-pipe and stops the vaporization of the 
alcohol. The flame then goes out as with any gas burner when the 
supply of gas is cut off. 

219. Precautions. Remember that alcohol is very inflam- 
mable, therefore special care should be exercised that none of it 
overflows from the reservoir or leaks from poor joints. It is per- 
fectly safe in burning through the burner, but any alcohol outside 
the lamp is dangerous, for if it catches fire it cannot be extinguished 
unless one has plenty of sand or non-inflammable dust to throw on 
it and smother the flame, or one of the modern chemical fire extin- 
guishers. 

TROUBLES IN CHAPTER V 

220. The prime difficulty with these relatively weak lights 
is the dim screen pictures. That is, they will be dim in comparison 
with the bright pictures obtainable with the direct current arc 
light. 

Remember the conditions requisite for screen images with weak 
lights ( 193). 

221. Smoking of the petroleum lamp or of the acetylene 
burner. This shows that the wicks are not properly trimmed or 
that they are turned up too high for the height of the chimney. 



134 TROUBLES IN CHAPTER V iCn. V 

With the acetylene flame if too much gas is turned on the flame 
will smoke and roar. 

222. The image of the lamp flame may show on the screen. 
This is because the objective is too far from the condenser or the 
lamp flame is not in the proper position with reference to the con- 
denser. Try removing the lamp farther from the condenser or 
bringing it nearer. When it is in the correct position its image 
will not appear on the screen. 

223. Roaring of the alco-radiant lamp. If the roaring is 
excessive it shows that the pressure on the alcohol reservoir is too 
great. This can be remedied by ceasing to pump the air in 
till the noise is within reasonable bounds. 



CH. V] 



DO AND DO NOT IN CHAPTER V 



135 



224. Summary of Chapter V: 
Do 

i. For these relatively weak 



sources of light use a good 
screen, and make the room dark 

( 



2. Use 
slides. 



transparent lantern 



3. The objective to select is 
one of large aperture for these 
large sources ( 217, 855). 

4. Have perfect containers 
for liquids and gases so that 
none can escape into the room. 



5. For the petroleum light 
and the alco-radiant use the 
objective shield (fig. 62) as it 
takes so long to get a good light. 



Do NOT 

1. Do not try to give an ex- 
hibition with these weak lights 
in a room with much stray light, 
and do not use a dirty screen. 

2. Do not try to use opaque 
lantern slides. 

3. Do not use an objective of 
small aperture with these large 
sources. 

4. Do not use leaky con- 
tainers for the gases or liquids 
used in this chapter. They are 
all dangerous when out of their 
proper containers. 

5. Do not turn off the alco- 
radiant or the petroleum light 
during the exhibition; it takes 
too long to start them. 



6. Follow carefully the direc- 
tions sent with the apparatus 
by the manufacturers. 



6. Do not fail to read care- 
fully and follow strictly the 
directions sent out by the manu- 
facturers. 



7. Do your part with great 
care and even these weak lights 
will give good projection within 
their range of possibility, i. e., 
for a screen two to three meters 
(six to nine feet) square. 



7. Do not expect too much of 
these weak sources, but give 
them a chance to do their best. 



136 



DO AND DO NOT IN CHAPTER V 



[CH. V 



Do 

1. Use a good quality of 
petroleum (kerosene) . 

2. Keep the lamp clean, and 
the wicks properly trimmed. 



3. Use a chimney of the 
proper height for the flame. 

4. Turn the flame up as high 
as possible without having it 
smoke. 

5. The edge of the flames 
should face the condenser, the 
middle flame being in the axis. 



Do 

1. For gas use the best kind 
of mantles. 

2 . Make the connections with 
rubber tubing of good thickness 
and quality or flexible metallic 
tubing (fig. 60). 

Do 

1 . For acetylene use a proper 
burner and reflector, that is, 
one which is made by a reliable 
house that has proved its safety 
and excellence. 

2. Use a safe gas supply, 
such as a house supply or a 
prcstolite tank is best. 



Do NOT 

1. Do not use poor oil, it will 
not give a good light, and may 
explode. 

2. Do not let the lamp get 
dirty or the wicks burn with 
tails. Clean and trim. 

3. Do not use a low chimney 
for a large, high flame. 

4. Do not turn the wicks up 
till they smoke. Stop just 
before that. 

5. Do not have the face of the 
flame, but the edge toward the 
condenser. 



Do NOT 

1 . Do not use mantles of poor 
quality, or that are broken. 

2. Do not make connections 
with thin or used up rubber 
tubing. 



Do NOT 

1. Do not use an untried 
lamp and general outfit for the 
acetylene light. Acetylene is a 
good servant but a cruel master. 

2. Do not try to use a make- 
shift gas generator. The smell 
will be disagreeable and the 
escaping gas possibly dangerous 



CH. V] 



DO AND DO NOT IN CHAPTER V 



137 



3 . Use thick and good quality 
rubber tubing or flexible metal- 
lic tubing (fig. 60) to make the 
connections. 

4. Study carefully the direc- 
tions for the use of the acetylene 
outfit with the magic lantern 
sent out by the manufacturers. 

5. Use perfect burners with 
the gas turned on sufficiently, 
but not enough to blow. 

6. Keep all naked lights away 
from an acetylene supply. Use 
an electric torch light if a light 
must be used. 



3. Do not use poor rubber 
tubing for connections. 



4. Do not neglect the careful 
study of the directions for using 
the acetylene outfit with the 
magic lantern. 

5. Do not try to use broken 
burners, and do not turn the gas 
on until it blows. 

6. Never let any naked lights 
come near an acetylene gas sup- 
ply. 



Do 

i . For the alcohol light , follow 
with care the directions accom- 
panying your alco-radiant lamp. 
Alcohol is dangerous stuff and 
should not be trifled with. 



Do NOT 

i. Do not fail to follow with 
scrupulous care the directions of 
the manufacturers of the lamp 
you use. 



CHAPTER VI 
THE MAGIC LANTERN WITH SUNLIGHT: HELIOSTATS 

230. Apparatus and material for Chapter VI: 

Suitable room for projection, preferably one with southern 
exposure ; Screen of proper size ; Porte- Lumiere or hand-regulated 
heliostat; Heliostat with clock-work for regulation; Condenser 
for bringing the parallel rays of sunlight to a focus (plano-convex 
or achromatic combination) ; Slide-carrier and projection objective. 

See also Ch. I, i. 

231. Historical. 

For the history of the magic lantern and all other projection 
apparatus with sunlight, see the Appendix. 

For Foucault's clock-driven heliostat see his: Recueil des 
Travaux Scientifiques, 1878, pp. 427-433. 

For the Heliostat of Mayer, using a lens and prisms, see Amer. 
Journal of Science, IV Ser. Vol. IV, (1897), pp. 306-308. 

For the Heliostats of Fuess, see C. Leiss, Die Optischen Instru- 
mente der Firma R. Fuess, 1899, pp. 284-305. For Heliostats 
like fig. 82, see Ambronn's Handbuch Astron. Instr. p. 649, fig. 637. 

Dolbear. Art of Projecting. 

LIGHT FROM THE SUN 

232. The limitless supply of light from the sun would be used 
in preference to any artificial source if it were only always avail- 
able. In many regions it is available during most of the year, and 
will no doubt be much more utilized as time goes on. Its use is 
strongly recommended in sunny regions. 

The sun is the brightest known source of light. Its intrinsic 
brilliancy is, in round numbers, 421,000 candle-power per square 
centimeter (2,720,000 candle-power per sq. inch). (See 23 2a). 

Sttnlight also serves as the standard for color values. 



232a. The intrinsic brilliancy of the sun. The intrinsic brilliancy of a 
source can be determined if its area and its candle-power are known. With the 
sun it is in convenient to make the reckoning in these terms as both the candle- 
power and distance are so enormous. The light from the sun near the zenith 
in clear weather amounts to 288,000 meter candles, that is, the sunlight is as 
powerful as the illumination due to 288,000 standard candles at a distance of 
one meter. (A. Arrhenius Lehrbuch der kosmischen Physik). 

138 



CH. VI] 



HELIOSTATS FOR THE MAGIC LANTERN 



139 



233. Heliostat. From the rotation of the earth on its axis 
from west to east the sun seems to move over the face of the sky 




\ 



FIG. 74. MAGIC LANTERN WITH SUNLIGHT. 

5 Sunlight. 

Mirror The plane mirror serving to direct the sunlight horizontally into 
the condenser. 

Condenser The single plano-convex lens serving to converge the parallel 
beam of sunlight. (Compare the second element of the condenser in fig. 2). 

Ls Lantern slide. 

Objective The projection objective for projecting an image upon the white 
screen. The projection objective and the condenser should be of approxi- 
mately the same focus. 

c Center of the projection objective where the rays from the condenser 
should cross. 

Axis Axis The principal optic axis of the condenser and of the projection 
objective. 

Screen Image The image of the lantern slide upcn the white screen. 



The apparent diameter of the sun's disc is 32'36" in midwinter and 31' 32" in 
midsummer, or it averages 32' 04" (Abbot, The Sun, p. 3; Ball's Astronomy, 
p. 127). 

The apparent area of the sun's disc at a distance of one meter is determined 
as follows: Its diameter is 32'o4" or .5343. One centimeter at a distance of 
one meter subtends an angle of .573, hence at one meter the sun's disc would 

appear to have a diameter of i = .933 centimeters. The area of such a 

573 

circle is : ' \ IT = .684 square centimeters. 
4 

. . .,.,,. Candle-power 

1 he intrinsic brilliancy is then, - 

Area 



288,000 

- = in round num- 



.684 

bcrs, 421,000 candle-power per square centimeter or 2,720,000 candle-power per 
square inch. 



140 HELIOSTATS FOR THE MAGIC LANTERN [Cn, VI 

from east to west. In order that the sun's rays may shine in one 
place continuously it is necessary to counterbalance in some way 
the apparent motion of the sun. 

If one holds a plane mirror in the hands, it is possible to keep a 
spot of sunlight on one place indefinitely by making slight changes 
in the position of the mirror to correspond with the changes in 
apparent position of the sun. This is possible from the law of 
reflection: "The angle of incidence and the angle of reflection are 
equal." (See fig. 80 and Chap. XIV, 794). 

A heliostat is then simply a mechanism for holding the mirror 
so that the sun's rays may be reflected in a constant direction. As 
this seems to make the sun stand still, the name is appropriate. It 
was given by the original inventor, s'Gravesande, 1742 (fig. 77). 

There are three principal forms of heliostats : 

(1) The hand-regulated heliostat or porte-lumiere with one 
mirror and a double movement up and down, and on the axis, so 
that it may be made to follow accurately the sun's apparent motion 

(fig. 75). ' 

(2) A heliostat with one mirror, in which the movements of the 
mirror are brought about by clock-work (fig. 77-79). 

(3) A Heliostat with two mirrors. One mirror is attached to 
the end or side of the clock-shaft. The other mirror is not con- 
nected with the clock-work. The second mirror serves to reflect 
the beam from the movable mirror in the desired direction, and 
is set by hand, once for all, at the beginning of the experiment. 

The clock-shaft rotates once in 24 hours with the single mirror 
heliostats (fig. 77-79), and also with the two mirror heliostat 
with the mirror at the end of the clock-shaft (fig. Si). 

When the mirror is attached parallel to the clock-shaft (fig. 82), 
the clock-shaft rotates once in 48 hours (fig. 82, A, B, C). 

INSTALLATION AND USE OF A HAND-REGULATED HELIOSTAT OR 

PORTE-LUMIKRE 

234. The hand-regulated heliostat or portc-lumiere consists 
of a plane mirror so mounted that it can move on two axes. The 
mirror should be about 15 x 30 cm. (6 x 12 in.) in size and sup- 



CH. VI] 



HELIOSTATS FOR THE MAGIC LANTERN 



141 



ported by a framework. This frame should be hinged so that it 
can be moved from the horizontal up to the vertical position. It 
must also be so mounted that it can be rotated around at right 
angles to the hinge motion. 




FlG. 75. PORTE-LUMIKRE WITH PLANO-CONVEX LENS AND PROJECTION 

OBJECTIVE. 
(Cut loaned by the C. II. Stoelting Co.}. 

The mirror for the reflection of the sunlight into the condenser is moved as 
necessary by the milled head just above the condenser. 

The two movements arc made by hand as often as needed while 
one is using the apparatus. In the original forms of Cuff and 
Adams (1744-1746, fig. /5a), and in some modern forms, there are 
two handles or milled heads extending into the projection room; 
with one of them the operator can raise or lower the mirror on its 
hinges, and by the other he can rotate it on the other axis. In the 
form here shown there is but one handle. This serves as a crank 



142 



HELIOSTATS FOR THE MAGIC LANTERN 



[CH. VI 



to turn the mirror around in a circle on its axis, and as a screw by 
means of which it is raised or lowered on its hinges (fig. 75). 

235. Setting up the hand-regulated heliostat. The appara- 
tus must be so placed that it receives the full sunshine on the 
mirror. 

In the forenoon an eastern exposure can be used, and in the 
afternoon a western one; or a southern one nearly all day. In 
practice a person will naturally use the window best adapted to his 
particular needs if he has a choice. 




SOLAR, PROJECTION MICROSCOPE OF ADAMS, WITH 
PORTE-LUMIERE. 

(From Adams' Essays, 1771, PI. VI). 

Fig. 4 shows the movable mirror (K-L) placed outside the shutter in the sun, 
0-P, screws in the square plate to fasten the instrument in the shutter; M-N; 
thumb screws by which the mirror is turned to hold the sun's rays in the right 
direction. The large tube, A-C-D, contains the condenser and receives the 
shorter tube, fig. 5. Fig. 5 shows the tube into which the objectives are fixed. 
If for large objects the lens (fig. 6) is screwed into the end, g, for smaller objects, 
the objectives are arranged in a piece (fig. 8) sliding into the opening at q. 
Notches along the objective slider indicate when the lens is centered. The 
specimen to be examined is inserted at h. For high powers the substage con- 
denser shown at fig. 7 is put in the tube between d-h. At b is a rack and pinion 
for focusing the object. 



CH. VI] HELIOSTATS FOR THE MAGIC LANTERN 143 

236. Darkening the room. The room is darkened in the 
usual way with curtains or shutters. The window where the 
apparatus is to be placed must be darkened by a shutter or a cur- 
tain with a hole in it, through which the instrument may be ex- 
tended out into the sunshine and through which the sunshine 
can be reflected into the room. 

The window frame must either be raised entire or one of the 
panes must be hinged so that it can be opened when desired. One 
can use the heliostat within the room utilizing the sunlight passing 
through the window glass, but this is far less satisfactory than hav- 
ing the heliostat out in the free air where the sun shines directly 
upon it. 

Finally it must be possible to close the openings completely so 
that the room may be made as dark as desired. 

237. Operation of the apparatus. In starting work at any 
time the mirror is inclined on its hinges and rotated until the sun 
shines upon it, and then until the light is reflected into the con- 
denser. Finally some further slight changes may be necessary to 
get the light accurately centered so that it will pass from the 
condenser along the common axis to the objective and thence to 
the screen (fig. 74). By changing the position of the mirror 
slightly every three to five minutes to compensate for the apparent 
motion of the sun, the light will continue to pass through the magic 
lantern to the screen. 

238. Adjustments necessary for the different windows. 

(A) For a southern exposure For this exposure it is desirable 
to have the entire outfit in a north and south direction with the 
objective pointing toward the north. In the morning the mirror 
is turned on its hinges to about 45 and then rotated toward the 
east until it receives the light of the sun (fig. 76). It must then 
be turned slightly by one or both of its possible movements until 
the light is reflected in the desired direction. As the sun continues 
to rise in the sky the mirror must be rotated on the axis from the 
east toward the west to follow the apparent movement of the sun. 
As the sun gets higher and higher the mirror must be turned on its 



144 



HELIOSTATS FOR THE MAGIC LANTERN 



[Cn. VI 



hinges more and more until at noon it will be nearly horizontal 
(fig. 76). In the afternoon, as the sun moves toward the west, the 
mirror must be rotated to follow it. At the same time it must be 
turned more and more on its hinges until late in the afternoon, it 
will be at the same angle as in the morning, and rotated as far 
toward the west as it was toward the east in the earlier part of the 
day (fig. 76). 

(B) For an eastern exposure In this position, the axis of the 
entire instrument is preferably east and west with the objective 
pointing westward. The earlier the time the more nearly hori- 




North 



FIG. 76. DIAGRAM SHOWING THE POSITION OF THE MIRROR NECESSARY 
TO RELFECT THE SUNLIGHT DIRECTLY NORTH AT THREE DIFFERENT 
PERIODS OF THE DAY (6A.M.; 12 M.; 6 p. M.). 

The diagram is for the latitude of Ithaca and at the season of the equinox 
when the sun seems to rise directly in the east and set directly in the west. In 
the morning thu mirror is turned toward the east at an angle of 45 and inclined 
about 10 toward the south. In the evening it is turned similarly toward the 
west and south. 

At noon the mirror is raised on its hinges about 28 above the horizontal. At 
all intermediate points the mirror must be set accordingly: that is, so that it 
will reflect the sun directly north. 

The diagram also shows the apparent course of the sun from sunrise to 
sunset. 



Cn. VI] HELIOSTATS FOR THE MAGIC LANTERN 145 

zontal must the mirror be. As the sun gets higher and higher the 
mirror must be raised more and more on its hinges ; and as the sun 
seems to move toward the south as well as upward, the mirror must 
be rotated on its axis toward the south. 

(C) For a western exposure If a western exposure is used, 
the entire instrument should be placed pointing east and west if 
possible. The mirror will be raised on its hinges and turned south- 
ward early in the afternoon. As the sun sinks toward the west the 
mirror will be made more and more nearly horizontal, and as the 
sun seems to move toward the north as well as toward the west, 
the mirror will finally be nearly horizontal on its hinges and rotated 
somewhat northward. 

These movements of the mirror become intelligible if one 
observes the position of the sun in the different periods of the day. 
By consulting fig 86, 87, it is also clear that the mirror must have 
different positions owing to the declination or position of the sun 
with reference to the horizon at different times of the year. 

HELIOSTATS DRIVEN BY CLOCK-WORK 

239. Types of clock-driven heliostats. A fundamental 
character of all heliostats is that the clock-work rotates a shaft 
corresponding with the post carrying the hour hand of an ordinary 
clock, and that it is this shaft which directly or indirectly gives 
motion to the mirror. 

This shaft must be made parallel with the earths axis wherever 
the instrument is used. 

(A) Single-mirror type. This is so constructed that the clock- 
work gives a double motion to the mirror something as one can 
give a double motion to a mirror held in the hands, i. e., an up and 
down motion and a motion of rotation on the axis (fig. 77-79). 

(B) Double-mirror type. In this type one mirror is fixed at 
the end, or the side of the clock-shaft. The second mirror is not 
moved by the clock-work, but is set by hand at the beginning of 
each experiment (fig. 81-84). 

As one might conclude, the second or two-mirror type is of 
simpler construction and therefore correspondingly inexpensive. 



146 



HELIOSTATS FOR THE MAGIC LANTERN [Ca. VI 




FIG. 77. ONE-MIRROR HELIOSTAT OF S'GRAVESANDE. 
(From his: Elementa Mathematica Phy sices, Tomus II, Tabula LXXXIII). 

This picture is a facsimile except that the clock-shaft has been extended 
above and below. 

B B Base of the mirror support with leveling screws. 

P Hollow cylinder in which the mirror support C can rotate. 

C The pillar with mirror fork at the upper end. 

5 Plane mirror. 

D K Shaft at right angles to the mirror and extending to the fork at the 
end of the clock-hand (N 0). 

L L M Foot of the support for the clock-work. 

Ill The leveling screws of the support. 



CH. VI] HELIOSTATS FOR THE MAGIC LANTERN 147 

G F Column supporting the clock-work. 

N P L The clock-shaft. It is parallel with the earth's axis and hence 
points toward the celestial north pole. The angle L at the lower end is equal 
to the latitude of the place where the heliostat is used. 

T R Fork where the movement of the clock-hand (N 0) is transferred to 
the shaft actuating the mirror (D E S). 

f g Plate bearing the clock-work. It must be elevated sufficiently to make 
the angle of the clock-shaft equal to the latitude of the place (see fig. 85). 

It answers very well for all the work required by the photographer 
and the projectionist. 

240. How to make the clock-shaft parallel with the earth's 
axis at any given place. For this it is necessary to know two 
things : 

(1) One must know the north and south direction. 

(2) One must know the latitude of the place. 

The first information can be gained by referring to the pole star. 
Buildings are often set due north and south, and thus serve as 
guides; or one might use a compass. If a magnetic needle is used 
it must not be forgotten that there is a certain variation from the 
true north and south line assumed by the compass needle, and for 
accurate observations it is necessary to know the magnetic varia- 
tion at any given place and to correct for it. 

For the latitude, a good map like that issued by the U. S. geologi- 
cal survey will give the information. The geological survey maps 
also give the magnetic variation. 

Making the clock-shaft parallel with the earth's axis is easily 
accomplished if one knows the latitude and the north and south 
direction. As a general statement all that is necessary is to make 
the clock-shaft point toward the north star or more accurately, 
toward the celestial north pole. 

By referring to fig. 85 it is evident that this is brought about by 
putting the instrument due north and south and then elevating the 
clock-shaft above the level or horizontal line an amount equal to 
the latitude of the place. 

For example, if an experiment with the heliostat is to be made 
in one of the buildings of Cornell University at Ithaca with, in 
round numbers, a latitude of 42.5 degrees, the instrument is set on 
a level place and due north and south, then the free end of the 



148 



HELIOSTATS FOR THE MAGIC LANTERN [CH. VI 




FIG. 78. ONE-MIRROR HELIOSTAT OF FOUCAULT. 
(From his Recueil des Travaux Scientifiques). 

Modified by extending the clock-shaft above and below. 

B Clock-work. 

T The clock-shaft. 

C The upper end of the clock-shaft. 

N P Continuation of the clock-shaft above the mirror (M). 

L The angle above the horizontal to make the clock-shaft parallel with the 
earth's axis. It equals the latitude of the place where the heliostat is used (see 
fig- 85). 

D Divided semicircle to enable one to set the instrument according to the 
sun's declination. 

N 1 N, N C l Connections between the clock-work and the mirror. 



clock-shaft is raised above the level 42.5 degrees. If now one were 
to sight along the clock-shaft it would be found pointing directly 
toward the north star. 

As seen from the diagram, fig. 85, it will then also be parallel 
with the earth's axis. 



CH. VI] HELIOSTATS FOR THE MAGIC LANTERN 149 

Sometimes the co-latitude and the vertical line are used instead 
of the horizontal line and the latitude. This brings about the same 
result, for if the clock-shaft is vertical to start with, it must be 
tipped over toward the north from the vertical an amount equal to 
the co-latitude. That is, in Ithaca, it must be inclined 47.5 degrees 
from the vertical. 

In general the clock-shaft must be inclined upward from the 
horizontal, a number of degrees corresponding with the latitude at 
the place of observation or it must be inclined downward toward 
the north from the vertical position a number of degrees correspond- 
ing with the co-latitude. The sum of the latitude and the co-lati- 
tude in every case equals 90 degrees. (See fig. 85 and its explana- 
tion) . 

INSTALLATION OF A SINGLE-MIRROR HELIOSTAT 
241. Setting up the heliostat. 

1. In the first place the instrument must be leveled and 

arranged accurately in a north and south direction. 

2 . The clock-shaft must next be elevated to an angle correspond- 

ing with the latitude of the place where it is to be used 
( 240) so that it will point toward the north star. It will 
then be parallel with the earth's axis (fig. 85). 

3. To give the proper angle to the mirror, depending on the 

declination of the sun, and to get also the correct local 
time, loosen the clamp holding the clock-arm (fig. 79 c) 
and turn the clock-arm toward the sun until the light 
shines through both sights along the line q-p. Then 
clamp the set screw at c. 

4. To get the spot of light in the desired place, loosen the clamp- 

screws in the position arm F-B and below H in the rotating 
collar and then raise or lower the shaft o, fig. 79 and rotate 
the position arm around the column A till the light is 
reflected where it is wanted, then tighten the clamping 
screws and the clock-work should cause the mirror to move 
so that it will reflect the beam of light in the same place so 
long as the sun shines on the mirror. 



HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI 




FIG. 79. UNIVERSAL, ONE-MIRROR HELIOSTAT. 
(From the Catalogue of R. Fuess). 

Modified by extending the clock-shaft above and below and by adding the 
abbreviations in and rf for the incident and reflected ray on the mirror (M). 

This heliostat is called universal for it is adjustable so that it can be used in 
any latitude and at any season of the year. See fig. 80 and 241 for further 
explanation. 

The dials showing the time and declination may be used for 
setting the heliostat, but one can get the apparatus set accurately 
by trial as just described. If the time and declination scales are 
to be used one must consult a nautical almanac for the sun's 
declination for the given date, and an accurate clock for the time 
of day. 

242. For centering the magic lantern when a heliostat is used 
the same general principles must be followed as with the arc light 
magic lantern (Ch. I, 51-57, fig. i, 74). 

To center the light one must be able to adjust the mirror by hand 
after it has been set to follow the sun. This is provided for in all 



CH. VI] HELIOSTATS FOR THE MAGIC LANTERN 151 

forms of single-mirror heliostats. In fig. 79, for example, the 
position arm B-F can be raised or lowered and the entire arm can 
be rotated around the column A. When the light is accurately 
directed, all the clamps can be tightened and the clock-work should 
cause the mirror to hold the light constantly in position. It will 
be found much easier to center the light on one axis if the heliostat 
is at about the same level as the condenser and objective. This 
position can be secured by raising the heliostat or the lantern, 
whichever is more convenient, provided the two are not on the 
same level to start with. 




o 
FIG. 80. PRINCIPLE OF THE UNIVERSAL HELIOSTAT SHOWN IN FIG. 79. 

O A The clock arm pointing directly towards the sun. 

B The position arm, pointing in the direction in which it is desired to 
reflect the light. 

in The incident light parallel to A. 

rf The reflected light. 

A B The mirror. The mirror is perpendicular to the plane passing through 
A , and B. 

N Perpendicular to the mirror A B. 

In order to prove that incident light parallel to A will be reflected from 
the mirror parallel to B it is necessary to prove that A O, B and N are 
in the same plane and that O N bisects the angle A B. The mirror being 
perpendicular to the plane containing A , and B and the line O N perpendicu- 
lar to A B must also be in this same plane. The triangle A O B is isosceles by 
construction, as A O and B are made equal, hence the perpendicular to the 
base must bisect the vertex angle. 



15.2 HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI 

INSTALLATION AND USE OF TWO-MIRROR HELIOSTATS 

243. Heliostat with the mirror at the end of the clock-shaft. 

Place the heliostat in a position either inside a room or outside a 
window where the full light of the sun can fall upon the movable 
mirror. The stand supporting the clock-work, etc., must be made 
level, and set in a north and south direction (fig. 81). 

Elevate the clock-shaft above the level to an angle equal to the 
latitude of the place where it is to be used. One can use a good 
protractor for this. The clock-shaft will then point toward the 
north star, and be parallel with the earth's axis (fig. 85). 

This form of heliostat often has the clock-shaft in a fixed position 
for cheapness of construction (fig. 81). If such a heliostat is 
purchased, the manufacturer must know the latitude of the place 
where it is to be used, then he will give the proper inclination to the 
clock-shaft so that when the instrument is arranged in a north and 
south line the shaft will point toward the north star. 

244. Arranging the movable mirror. The mirror is fixed to 
the end of the shaft by a collar which permits it to rotate around 
the shaft. It is also held in a kind of fork, which permits the 
mirror to be raised and lowered in a way similar to the hinge 
movement of the porte-lumiere (fig. 75). 

For setting this mirror so that the clock-work will cause it to 
throw a beam of light in one direction continuously, it is necessary 
first of all to set the mirror for the local time. This is done by the 
use of a perforated screen admitting a narrow pencil of light from 
the sun. This screen is so placed that the pencil of light falls upon 
the mirror. The mirror is then turned by loosening the clamp 
(fig. 8 1 c) and rotating it on the shaft, and by tipping it in the fork 
until the pencil of light is reflected back along its path through the 
hole again. 

Then the clamp is tightened and the screen removed. The 
mirror is now tipped in the fork until the light is reflected from it 
directly in line with the clock-shaft, i. c., directly toward the north 
star (fig. 81 N. P.). The easiest way to do this is to take a piece 
of white cardboard with parallel black lines on it and place it 



CH. VI] 



HELIOSTATS FOR THE MAGIC LANTERN 



153 




FIG. 81. TWO-MIRROR HELIOSTAT WITH THE MOVABLE MIRROR AT THE 

END OF THE CLOCK-SHAFT. 
(From the Catalogue of Max Kohl). 

The figure has been modified by extending the clock-shaft and by adding the 
second mirror and the light rays. This heliostat is usually fixed for a given 
latitude, hence in ordering one, the latitude of the place should be given. It 
could be made adjustable for latitude, but that would naturally increase the 
cost. 

N-P, L The clock-shaft pointing to the celestial north pole above, and 
indicating the angle corresponding with the latitude below. 

c Clamp to hold the mirror (M) to the revolving clock-shaft. 

M Movable mirror. It is adjusted in the fork and on the clock-shaft until 
the reflected rays proceed parallel with the earth's axis, hence also parallel with 
the clock-shaft. 

M 2 The fixed mirror to be set by hand in the beginning. 

S and the Arrows. The sun's rays. 



154 HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI 

parallel with the clock-shaft. When the beam of light from the 
mirror extends out parallel with these lines, as indicated by the 
streak of light, the mirror will be in the correct position. 

245. Arranging the second mirror. For getting the light in a 
desired direction, a second mirror is used in the path of the beam 
extending directly northward, from the first mirror, and so arranged 
that the light is reflected as desired (fig. 81 M 2 ). 

246. Heliostat with the mirror parallel with the clock-shaft. 

With the other heliostats described in this chapter, the clock-work 
rotates the shaft once in 24 hours, but with this form, the rotation 
is once in 48 hours, i. e., half the rate of rotation of the earth. The 
clock-shaft is somewhat extended and the mirror is fixed directly to 
the shaft and parallel with it. The mirror is therefore in a plane 
which if extended would cut the celestial north pole (fig. 82). 

Light reflected from this mirror may be made to take any direc- 
tion in a circle. 

247. Setting up the heliostat with the mirror parallel with the 
clock-shaft. The heliostat is placed in a proper position for 
receiving the sunlight. The support is made level, and the instru- 
ment set north and south. The clock-shaft is then elevated from 
the horizontal until it is at an angle equal to the latitude of the 
place where it is to be used. As the mirror in this form may be set 
to reflect the light anywhere in a circle, it is best to loosen the clamp 
of the clock-shaft and rotate the mirror until it receives the full 
light of the sun and reflects it in a convenient direction. Then 
clamp the shaft to the clock-work and the mirror will follow the 
sun. 

248. Arranging the second mirror. The second mirror is now 
placed so that it will receive the beam from the movable mirror, 
and then it is turned, raised, or lowered on its stand, until the light 
extends in the desired direction. It should continue to hold the 
light in one place so long as the sun shines on the movable mirror 
(fig. 82). One must make sure that the position of the second 
mirror is such that it will not shade the heliostat mirror as the 
sun moves toward the west. 



CH. VI] 



HELIOSTATS FOR THE MAGIC LANTERN 




FIG. 82. 



TWO-MIRROR HELIOSTAT WITH THE MOVABLE MIRROR ATTACHED 
PARALLEL TO THE CLOCK-SHAFT. 



This heliostat is adjustable for latitude and can be used anywhere in the 
northern hemisphere, and by reversing the motion, in the southern hemis- 
phere ( 253). 

C Clock-work mounted on a hinged plate. 

M 1 Rotating mirror attached to the side of the clock-shaft. From this 
arrangement its plane would pass through the celestial north pole if extended. 



*To get this picture, the heliostat was set in the west window of Stimson Hall 
at 2:30 P. M., May 20, 1912, and the mirrors arranged to receive and reflect a 
small beam of sunlight as indicated. A black cord was extended from the 
small hole in the shutter to the point on the first mirror receiving the sunbeam, 
and from thence to the second mirror along the path of the sunbeam ; and from 
the second mirror to a point on the screen receiving the sunbeam. The 
apparatus was then photographed. To make the course of the sunbeam very 
sharp for this cut its course was traced on the photograph by a right line pen. 
The clock-shaft was also extended above and below and an arc of a circle added 
between the clock-shaft and the horizon to indicate the angle of elevation of 
the clock-shaft, corresponding with the latitude of Ithaca (42.5 North Lati- 
tude). 



156 



HELIOSTATS FOR THE MAGIC LANTERN 



[CH. VI 



M 2 The fixed mirror. This is adjusted at the beginning of the experiment 
to reflect the light in the desired place and usually needs no attention during 
the experiment. 

N P Continuation of the clock-shaft pointing toward the north pole. 

L 42.5 The angle made by the clock-shaft, and the horizon at Ithaca, 
New York, U. S. A. It indicates the latitude of that place, and the elevation 
of the clock-shaft to make it point toward the celestial north pole. 

S B Sunbeam admitted through a hole in the shutter. It strikes the first 
mirror and is reflected to the second mirror, and from it in any desired direction. 



\2 M 





B 



FIG. 82 A, B, C. DIAGRAMS SHOWING THE POSITION OF THE FIRST 
MIRROR OF THE HELIOSTAT (Fig. 82) AT DIFFERENT TIMES OF THE DAY 
TO REFLECT THE SUNLIGHT CONSTANTLY IN THE SAME DIRECTION. 

The eye is supposed to be looking along the axis of the clock-shaft. It is to 
be noted that between 6 A. M. and 6 P. M. (12 hours) the mirror has turned 
through an angle of 90, and at this rate it takes 48 hours for the mirror to 
make a complete revolution of 360. 

The arrows indicate the direction of the light before and after reflection from 
the mirror. 

A Position of the mirror at 6 A. M. 

B Position of the mirror at 12 M. 

C Position of the mirror at 6 P. M. 

At intermediate periods the mirror will be in correspondingly intermediate 
positions to reflect the sun constantly in the same direction, that is, the mirror 
must follow the sun. 

This is one of the easiest heliostats to manage, as one needs to 
know only the latitude and the north and south direction. The 
arrangement of the two mirrors can be easily made at any time 
and in any place by trial. 



HELIOSTATS ix THE SOUTHERN HEMISPHERE 

249. U]) to the present, the discussion has been with reference 
to heliostats in the northern hemisphere. For those to be used in 
the southern hemisphere certain modifications are necessary as seen 
from the following considerations : 



CH. VI] HELIOSTATS FOR THE MAGIC LANTERN 



157 




FIG. 83. LENS AND PRISM HELIOSTAT OF ALFRED M. MAYER. 
(From the American Journal of Science, Vol. 154, iSg 1 ^). 

This heliostat is in principle like the two-mirror heliostat with the movable 
mirror attached to the end of the clock-shaft (fig. 81). 

J Biconvex lens about 10 cm. (4 in.) in diameter to receive the sun's rays 
and render them convergent. 

K Concave lens to render the converging beam parallel. 

g Rack and pinion movement to change the position of the concave lens 
and thus increase or diminish the size of the beam. 

/ Right-angled prism receiving the parallel bundle from K and reflecting it 
to a fixed prism (L) or to a mirror, by which it is reflected in any desired 
direction. 

The two lenses J K and the prism /, are all on one common axis and are 
rotated by the clock-shaft G, and thus made to follow the sun like the mirror 
on the end of the clock-shaft in figure 81. The clock-shaft G must be at an 
elevation corresponding to the latitude of the place (see also fig. 84). 



158 HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI 

250. If one were looking at the north pole of the earth from 
a position along the earth's axis, the direction of the earth's rota- 
tion would appear in a direction opposite to the hands of a clock or 
watch. To compensate for this, a mirror to hold a spot of sunlight 
in one position would need to be rotated around an axis parallel 
with that of the earth, but in an opposite direction to the earth's 
rotation, that is in the clockwise direction. 

251. At the equator, the clock-shaft must be horizontal in 
order to be parallel with the earth's axis. The clock-shaft must be 
turned from east to west. This can be accomplished either by a 
clock-work located at the southern end of the shaft turning in the 
clockwise direction as in fig. 77-79, or by a clock-work located at 
the northern end of the shaft turning in a counter-clockwise direc- 
tion. 

252. At the north pole of the earth, the axis of rotation of the 
shaft would be vertical and the direction of rotation as seen from 
above, would be clockwise. 

At the south pole the axis would also be vertical and the direction 
of rotation would be clockwise as seen from below i. e., from the 
north or counter-clockwise as seen from above. 

253. A heliostat constructed for the southern hemisphere 
would be exactly similar to one for the northern hemisphere except 
that the clock-shaft must rotate in the counter-clockwise direction, 
that is, from right to left. 

254. Setting up a heliostat in the southern hemisphere. If 

a heliostat is properly constructed for the southern hemisphere it 
is set up at any given south latitude by arranging the instrument 
due north and south with the free end of the clock-shaft pointing 
south. Then the clock-shaft would be elevated above the horizon 
a number of degrees corresponding with the south latitude. This 
would make the clock-shaft parallel with the earth's axis and it 
would point toward the celestial south pole (fig. 85). Indeed, the 
entire procedure for getting the light in the desired direction, the 
use of the condenser and projection objective, etc., is exactly 
as for the northern hemisphere. 



CH. VI] 



HELIOSTATS FOR THE MAGIC LANTERN 



159 





FIG. 84. LENS AND PRISM HELIOSTAT OF ALFRED M. MAYER. 
(From the Catalogue of Optical Instruments by R. Fuess), 

The figure has been modified by extending the clock-shaft above and below. 
As here shown the instrument is suitable for any latitude. It uses a mirror 
instead of a second prism as in the original of Mayer (fig. 83). 

U Clock-work. 

N P, L The clock-shaft extended to indicate the direction of the celestial 
north pole above, and below the angle of elevation corresponding to the lati- 
tude of the place where the instrument is used. 

P D Z Three divided scales; P for the latitude, D for the Sun's declination, 
Z for the time of day. 

S, k and Pr. The convex and the concave lens, and the prism as shown in 
fig- 83. 

Sp Mirror to take the place of the prism (L) in fig. 83. 



i6o 



HELIOSTATS FOR THE MAGIC LANTERN [Cn. VI 




FIG. 85. DIAGRAM SHOWING THAT THE ELEVATION OF THE CLOCK-SHAFT 
AT AN ANGLE EQUAL TO THE LATITUDE OF A PLACE WILL MAKE THE 
CLOCK-SHAFT PARALLEL WITH THE EARTH'S Axis. 

EQ Equator of the earth. 

Axis Axis The earth's axis with the north pole of the earth above and the 
south pole below. 

N P The earth's north pole. 

S P The earth's south pole. 

42.5 Latitude of Ithaca, New York, U. S. A. 

h h Horizontal lines, that is, tangents to the earth's surface at the two 
latitudes shown (42.5 north, 30 south). 

Z Zenith. 

A A Clock-shaft elevated from the horizon an amount equal to the latitude. 
If continued toward the equator the clock-shaft would meet the plane of the 
equator at right angles, hence it is parallel with the earth's axis and points 
toward the celestial poles. 

A h Latitude (42.5 north and 30 south). 

A Z Co-latitude (47.5 north, 60 south). 

255. Finally, a hcliostat constructed for the northern hemi- 
sphere would work equally well for the southern hemisphere if it 
were attached to the ceiling (i.e. wrong side up) instead of being on 
a table or window-sill right side up, for this change in position would 
make the clock-shaft rotate in the counter-clockwise direction, as. 
seen from above. 



CH. VI] CONDENSER FOR SUNLIGHT 161 

CONDENSER FOR SUNLIGHT 

256. As sunlight is composed of practically parallel rays, the 
condenser consists of a single plano-convex lens with the convexity 
receiving the light (fig. 74); or one may use an achromatic com- 
bination (fig.324). 

The condition is practically like the ordinary condenser after the 
light has been rendered parallel by the first element of the condenser 
(fig. 3). Having parallel rays to start with, only the second ele- 
ment of the condenser is needed. 

257. Increasing the illumination. The greatest difference 
between the use of sunlight and the arc light for projection appears 
when one wishes to increase the illumination. With the arc lamp 
one simply uses more current, and this increases the candle-power 
and makes the screen image more brilliant. With the same size 
condenser and picture the illumination of the screen with the arc 
light is directly proportional to the illumination of the condenser 
face. 

With sunlight, the illumination of the condenser face is a con- 
stant quantity except for haze, etc. As all the light which strikes 
the screen must pass through the condenser, the screen illumina- 
tion can be increased with sunlight only by using a condenser of 
larger diameter and correspondingly greater focal length. And for 
this one must have heliostat mirrors of sufficient size to fill the 
condenser with light. 

258. The water-cell with sunlight. This light is accom- 
panied by so much radiant heat that it is desirable to use a water- 
cell with the apparatus, and thus reduce the liability of over-heating 
lantern slides or other specimens used for projection (see 848 for 
the discussion of the need of a water-cell) . 

CONDUCT OF AN EXHIBITION WITH SUNLIGHT 

259. The general principles given in Ch. I, 21-41 are 
applicable. 

260. Lighting of the room. Sunlight is sufficiently powerful 
so that the room used need not be very dark for showing lantern 



162 USE OF SUNLIGHT FOR PROJECTION lCH. VI 

slides. Care must be taken to have no direct light fall on the screen 
except that from the lantern, but the room can have sufficient 
diffused light to take notes comfortably (see also Ch. XII, 
605-608). 

261. Size of the room and the screen. By using a condenser 
of proper size and of a focal length adapted to the projection 
objective, there is no practical limit to the possibilities of projection 
with sunlight. 

262. Turning on and off the light. For shutting out the sun- 
light one can use a metal shield between the mirror and the con- 
denser or one can use the objective shield (fig. 14 and 62). The 
first method is preferable, for there will be less heating of the 
apparatus. 

TROUBLES 
263. The troubles with sunlight are: 

1 . The difficulty of keeping the beam of sunlight in a constant 

direction. With the porte-lumiere one must be con- 
stantly on the alert to make the slight adjustments of the 
mirror necessary. 

2. The clock-driven heliostats, if well made and regulated 

accurately, should give no trouble when they are prop- 
erly setup. 

If a person is fortunate enough to live near an astronomical 
observatory and can get the help of the astronomer in charge he can 
learn to overcome difficulties that seem to be insurmountable when 
working alone. The apparatus of an observatory is also of first 
rate quality, and it helps any worker to know what good apparatus 
looks like. 

264. Lack of sunlight. This is the one great trouble. Of 
course it is not available at night anywhere. And in the most 
thickly populated regions where projection apparatus is used there 
is liable to be so much cloudy weather that sunlight is not available 
even in the daytime during much of the year. Smoke also obscures 
the sun when clouds are absent. 



CH. VI] 



TROUBLES WITH SUNLIGHT 



163 



Fortunately, in many parts of America the sun can be counted 
on in the daytime ; and for those parts the use of sunlight for 
projection of all kinds is strongly recommended. 



North Pole 




South Pole 



FIG. 86. DIAGRAM OF THE CELESTIAL SPHERE WITH THE PLANES OF THE 

CELESTIAL EQUATOR AND OF THE ECLIPTIC; AND WITH THE APPARENT 

POSITION OF THE SUN AT DIFFERENT SEASONS. 

Earth This is shown as a small black sphere at the center. 

North Pole, South Pole The two poles of the celestial sphere. They are at 
an infinite distance from the earth. 

West, East East and west points of the celestial sphere. The plane of the 
celestial equator touches these points. 

Equator The plane of the celestial equator (shaded in lines) dividing the 
celestial sphere into a northern and a southern hemisphere. A plane at right 
angles to this traversing the north and south poles would divide it into an 
eastern and western hemisphere. 

Ecliptic The plane (shaded in dots) around the outer edge of which the 
sun seems to move during the year. It is inclined to the equator at an angle 
of 23 27.' 

Equinox When the sun appears at the equator the days and nights are of 
equal length (March 21, Vernal or Spring Equinox, and Sept. 23, Autumnal or 
Fall Equinox). 

Solstice The point on the Ecliptic the farthest north or south of the Equa- 
tor. (Summer Solstice, when north of the equator, June 22 ; Winter Solstice, 
when south of the equator, Dec. 22). 

(See also fig. 87). 



164 



DO AND DO NOT WITH SUNLIGHT 



[Cn. VI 



265. Summary of Chapter VI: 

Do Do NOT 

i . Utilize sunlight when that i . Do not use artificial light 
is available, for it is the bright- in a region where bright sun- 
est light to be had on our planet light is constantly available. 
( 232). 



2. For sunlight some sort of 
a heliostat is necessary to 
counterbalance the rotation of 
the earth, and make the sun 
shine in one place continuously 
( 233). 

3 . Two motions to the mirror 
are necessary, an up and down 
motion and a rotary motion at 
right angles to this ( 233). 

4. If a clock-driven heliostat 
is used, the instrument must be 
set up so that the shaft of the 
clock shall point toward the 
celestial pole and thus be 
parallel with the earth's axis 
( 239-241). 

5. To make the shaft parallel 
to the earth's axis raise it from 
the horizontal an amount equal 
to the latitude of the place 
where it is to be used ( 240). 

6. The two-mirror heliostat 
is simplest and least expensive 
( 239). 



2. If a porte-lumiere is used 
to keep the sun shining in one 
place, do not forget to adjust 
the mirror frequently. Remem- 
ber that the earth never stops 
rotating. 



3-4. For a clock-driven helio- 
stat do not forget that the shaft 
moving the mirror must point 
toward the north pole (or 
south pole, if south of the 
equator) . 



5. Do not forget to elevate 
the clock-shaft an amount equal 
to the latitude of the place. 



6-7. Do not put the second 
mirror of the heliostat so that 
the sun cannot shine on the 
first mirror. 



CH. VI] 



DO AND DO NOT WITH SUNLIGHT 



165 



7. One mirror is attached to 
the shaft and is driven by the 
clock-work. The other mirror 
is set by hand at the beginning 
of the experiment ( 239, 248). 

8. As the rays of sunlight are 
practically parallel, only one 
element of the condenser is 
needed, viz., the one next the 
lantern slide (fig. 74, 256). 

9. To increase the illumina- 
tion use a larger mirror and 
condenser ( 257). 

10. To turn the light on and 
off, use a metal shield (262). 



n. Use a heliostat designed 
for the hemisphere where you 
are to work (249-255). 



8. Do not use a condenser 
with two or three lenses for sun- 
light as for a near light source, 
use only one lens or an achroma- 
tic combination designed for 
parallel light (fig. 74). 

9. One cannot increase the 
illumination without increasing 
the size of the mirror and con- 
denser. 

10. Do not use inflammable 
shields to block the light. Use 
a metal shield between the 
mirror and condenser. 

1 1 . Do not try to use a helio- 
stat in the southern hemisphere 
which was constructed for use 
in the northern hemisphere. 




270. Apparatus and material for Chapter VII: 

Suitable projection room with screen ( 286); Lantern with 
projection objective of large aperture and with suitable radiant 
and condenser ( 275, 277, 279, 294-296); Suitable objects for 
projection ( 285). 




FIG. 88. CAMERA FOR DRAWING LANDSCAPES. 
(From the Catalogue of Queen & Co., 1880). 

The dark room is made of opaque cloth over a tripod. The 45 mirror at 
the top rotates to take in any desired part of the surrounding country and an 
objective projects the image down upon the horizontal drawing shelf. 



If there is to be combined projection, a tinted glass to make the 
lantern-slide image as dim as the opaque image ( 282). 
See also the outfit given in i, Ch. i. 



1 66 



CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 167 

271. Historical development. See appendix. 

References to literature : See the books referred to in Ch. i, 2, 
also the special catalogues and directions furnished by the manu- 
facturers of Opaque Lanterns and combined projection apparatus. 



PROJECTION OF IMAGES OF OPAQUE OBJECTS 
272. All of the images seen on a white screen within a dark 
room were originally of opaque objects. These objects were 
brilliantly illuminated by the sun, and the light reflected from them 




FIG. 89. CAMERA FOR EXHIBITING SURROUNDING LANDSCAPES. 
(From the Catalogue of McAllister). 

In a kind of cupola at the top is situated a plane mirror and beneath that a 
projection objective. The cupola rotates, thus enabling the operator to bring 
any desired scene upon the horizontal screen within the room. Such cameras 
were once common at fairs and in parks. 



1 68 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII 



\ 




10 11 12 13 14 15 



FIG. 90. DIAGRAM SHOWING OPAQUE PROJECTION. 

In both these diagrams (fig. 90-91) the same amount of light illuminates the 
object, and the objects are of the same size, and the objectives have the 
same aperture. 

Fig. go. Opaque projection In. L. Incident light of parallel rays imping- 
ing upon a picture in white and black. 

Object The opaque object in black and white the size of a lantern slide. 

1-15 The beams of light illuminating the object. The light must of 
course fall upon the surface facing the projection objective. 

R L Reflected light. From each point on the surface of the opaque object 
the light falling upon it is reflected nearly equally throughout the entire hemi- 
sphere. 

Ax Axial beam on the principal optic axis of the objective. 

Objective The projection objective. Its aperture is such that it receives 
and transmits about 20 of the 180 reflected from each point. 

From the formula given in 857:1 such an objective transmits to form the 
screen image approximately 3% of the light reflected from the opaque object. 



CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 169 




FIG. 91. TRANSPARENCY PROJECTION. 

In L Incident light. This is supposed to be exactly the same as that 
striking the face of the opaque object. In this case it traverses the condenser 
lens, passes through the transparency, and the objective, and passes on to 
the screen with very little loss. 

1-15 Parallel beams of light reaching the condenser and passing onward. 

Condenser A plano-convex lens to render parallel rays converging. 

L S Transparent lantern slide. 

Ax The principal optic axis. 

Objective The projection objective. Its aperture is the same as in fig. 90, 
but is much larger than necessary for the transparency. 



I yo PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. .VII 

passing through a hole, or later a lens, in the wall of a dark room 
sufficed to produce the picture on the white wall or screen. 

Later it was found that it was possible to illuminate objects 
sufficiently with artificial light to get screen pictures; and still 
later transparencies were used ( 272a). 

Every one who looks at the picture of a landscape, etc., depicted 
on the ground glass of a photographic camera sees inverted images 
like those originally observed in darkened rooms on translucent 
screens. 



CONDITIONS FOR OPAQUE PROJECTION: COMPARISON OF PROJEC- 
TION WITH OPAQUE AND TRANSPARENT OBJECTS 

273. In order to deal intelligently and successfully with 
opaque projection it is necessary to comprehend in the very begin- 
ning the difference in the conditions for obtaining a screen image 
of an opaque object, and for a screen image of a transparency 
(lantern slide, moving picture film or microscopic specimen). 

With a transparent or semi-transparent object, the light comes 
from behind and traverses the object, and goes on with practically 
no variation in direction to the projection objective. As the light 
reaching the lantern slide or transparency is directed by the con- 
denser (fig. 91), the light which illuminates the transparency passes 
on and enters the projection objective and therefore serves for the 
production of the screen image (fig. 1-2). 

With the opaque object, on the other hand, all the light which 
produces the screen image must be reflected from the surface of the 
object, and the light which illuminates the object must strike its 



272a. In the early days of opaque projection with artificial light the whole 
face of a man was sometimes shown; this, of course, required very large lenses. 

This is what Hepworth says concerning these exhibitions: "At one time a 
large instrument of this type was made for casting the image of a human face 
on the screen, the lenses being of immense size. . . It was, of course, fitted 
with a reversing (erecting) lens (fig. 208), so that the face should appear right 
way up. The owner of this face, by the way, suffered tortures during the short 
time of exhibition, for the powerful lime lights close to and on each side of his 
head, were so hot that they blistered his skin. He was made to smile at the 
audience, and then to drink their good health in a glass of wine, a refreshment 
which the poor man really needed after his grilling." (P. 246). 



CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 171 

face instead of traversing it, that is, it must extend in the opposite 
direction from that used with the transparency. 

The light falling upon the face of the opaque object must then 
be reflected from each point. But unlike the tranteparent object, 
in which practically all of the light illuminating each point of the 
object goes directly to the projection objective (fig. 91), with the 
opaque object, each point reflects the light irregularly and in all 
directions within the entire hemisphere ( 1 80 degrees, fig. 90) . This 
being the case, only a part of the light reflected from each point can 
get into the projection objective, all the rest falling outside the 
objective. Of course, the larger and closer the objective, the more 
of the light will be received; hence, in selecting an objective for 
opaque projection, keep in mind that the greater the diameter of 
the lenses the more light from each point can be received, and con- 
sequently the more brilliant will be the screen picture. 

It is assumed in this discussion, and in the accompanying dia- 
grams (fig. 90-91), that the opaque object is black and white and 
that it and the transparent lantern slide are of the same size; 
that both are lighted by a similar beam of parallel light rays, and 
that none of the light is lost by absorption. 

274. Relative amount of light for the images with trans- 
parencies and opaque objects. If, for example, as in the diagram, 
the projection objective can receive but 20 degrees of the hem- 
isphere of light from each point, then 160 degrees will fall outside the 
objective and not aid at all in the formation of the screen image. 
If the objective could take in all of the light from each point, the 
opaque object would give as brilliant a screen image as the lantern 
slide, but the actual proportion of light represented by the angle of 
twenty degrees is only three per cent, of that represented by 180 
degrees. As only three per cent, of the light from each point helps 
in the formation of the screen image of the opaque object, the 
opaque object can give a screen picture only three per cent, as 
bright as the transparency where practically all of the light helps 
to form the screen image (fig. 90-91). 

In practice, how great a proportion of light serves for the screen 
image and how much is absorbed or lost depends upon the opacity 



172 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII 

of the lantern slide and the reflecting qualities of the opaque object 
(see 274a). 

275. Aperture of the projection objective for transparencies 
and for opaque objects. By comparing figures 90-91 it will be 
seen that for a transparency, relatively small aperture for the 
projection objective is sufficient. This also shows that if one were 
to use the same objective for both transparencies and for opaque 
objects, that the difference in brightness would be enormously 
exaggerated, if one used only the necessary aperture for the trans- 
parencies. If one used the proper objective for the opaque object, 
it would answer well for the transparency, but only a part of the 
aperture would be utilized. As the large aperture makes the 
objective very expensive, one wastes money by having the large 
aperture for transparencies In the best practice, an objective of 
moderate aperture is used for transparencies, and one of relatively 
very large aperture for opaque projection. 

276. As will be shown later (Ch. XIV, 8 5 ya), with a given 
object and a given illumination, the brilliancy of the screen image 
depends upon the aperture of the objective and its distance from 
the screen. The larger the diameter of the lenses of an objective 



274a. Light flux getting through the objective with opaque projection. 

It will be shown in 857a that the light received from a perfectly white, per- 
fectly diffusing surface is 

> 

vSin 26 d20 _ _ T B 

(i cos 20) 

o 

(i cos 20) lumens per square centimeter of the white reflecting 

surface, where I is the intensity of illumination of the surface measured in 
meter candles, and 6 is the half angle of light subtended by the objective, or 26 
is the angle of light subtended by the objective. The light received by the 
surface is I/io,ooo lumens and the proportion of light received by the surface 

i cos 2B 
which strikes the objective is then 

In this problem the angle of light subtended by the objective is 20, i. e. 
26 = 20. The proportion of light received by the objective is then (i cos 
20)/2 = (i -9397)/2 = .0603/2 = .0302 or about T, r " ( . 



CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 173 

of given focus, the greater will be the brightness. With the same 
objective, the greater the distance of the objective from the screen, 
the less will be the brightness 





FIG. 92. CHADBURN'S OPAQUE 

LANTERN WITH ONE SOURCE 

OF LIGHT. 

(From Chadwick, Hepworth and 
Wright). 

L Source of light shining directly 
upon the opaque object. 

M Beam of light from the opaque 
object to the objective and to the 
screen. 



FIG. 93. CHADBURN'S OPAQUE LAN- 
TERN WITH Two SOURCES OF LIGHT, i 
(From Chadwick, Hepworth and Wright). 

This form requires two sources of 
light and two condensers. The light 
is projected directly upon the object 
and from the object it extends out 
through the objective to the screen. 
This method is still often employed. 

The same lantern, connected in the 
usual way, was employed for trans- 
parency projection (fig. i). 
L-L Source of light and condenser arranged to send the light directly to 
the opaque object. 

D-D Hinged door for the support of the book, picture or other object. 
When the door is closed, the light from both sources shines directly upon the 
opaque object. 

B Beam of light from the object to the objective. 

A Objective of large aperture for projecting the image of the opaque object 
upon the screen. 



174 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII 

277. Brilliant screen images of opaque objects. It is intel- 
ligible from the above discussion and the diagrams that to produce 
a brilliant screen image of an opaque object five things are neces- 
sary: 

1. The light for illuminating the opaque object must be very 

brilliant, like sunlight or the electric arc light. 

2. The opaque object must be capable of reflecting most of the 

light illuminating it, or must be on a white background. 

3. The projection objective must have lenses of large diameter. 

4. The distance of the objective from the screen must not be 

too great. 

5. Besides the above, the projection room must be dark or the 

screen image will not have sufficient contrast. 




FIG. 94. DOLBEAR'S OPAQUE PROJECTOR WITH SUNLIGHT. 

(From Dolbear's Art of Projecting). 

H Heliostat, porte-lumiere or simply a plane mirror to direct the sunlight 
through the bi-convex condenser. 

r Movable mirror to reflect the sunlight upon the opaque object at d. 
The handle for changing the inclination of the mirror is seen at the right. 

d Opaque object with the light from the mirror (r) illuminating it. 

o Projection objective. 

S Screen for the image. 

278. Position of the radiant. The radiant or source of light 
for illuminating opaque objects for projection may have either of 
two positions: 



CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 175 

1. It may be in front of the object so that the light emitted 

shines directly on it. This is the original device and gives 
the greatest amount of light (fig. 92-93); or the radiant 
may be tilted (fig. 105, in). 

2. The second method is to have the light not in front, but a 

mirror reflects the light from the radiant upon the opaque 
object (fig. 94, 95). This is usually a more convenient 
arrangement than the above, but a certain amount of the 
light (between 10% and 25%) is lost when reflected 
from a mirror. 

279. Use of a condenser or concave reflector with opaque 
projection. This is frequently employed for the object is often at a 
considerable distance from the radiant, and too small a part of the 
light from the radiant would be available but for the help of the 
condenser. 

In most cases only the first element of the condenser is used. 
This projects upon the object or the mirror a cylinder of parallel 
rays (fig. 90,103). Sometimes also a converging lens of long focus 
is put in the path of the parallel cylinder to concentrate it more or 
less, depending upon the size of the object to be shown. Instead 
of a condenser, there is sometimes used a reflector (fig. 95, 96) 
behind the radiant. 

280. Darkness of the projection room. Owing to the diffi- 
culty of obtaining a sufficiently brilliant screen image it is necessary 
to have the projection room very dark. 

COMBINATION LANTERN SLIDE AND OPAQUE PROJECTION 
281. Daylight and twilight vision. Nearly all modern 
apparatus giving opaque projection also gives transparency pro- 
jection with a slight change. These two kinds of projection are 
mutually antagonistic for the adjustments of the eyes of the specta- 
tors. For transparency projection the image is so brilliant that 
the eyes are adjusted for daylight vision in large part, while for 
the opaque projection the image is so dim that the eyes should be 
adjusted for twilight or night vision. 



1 76 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII 



M 




ZEISS EPIDIASCOPE FOR OPAQUE AND FOR TRANSPARENT OBJECTS 
IN A HORIZONTAL POSITION. 

(Zeiss' Special Catalogue). 

As shown in this figure the apparatus is set up for opaque objects. For 
transparent objects M 2 (mirror 2) is removed when the light striking M 3 is 
reflected to M 4 and thence up through the object to M 1 and to the screen. 

Commencing at the right : R Parabolic reflector, which projects the light 
from the crater through (W) the water-cell to M 1 the mirror which is at the 
proper angle for reflecting the light down upon the opaque object. From the 
opaque object the light is irregularly reflected through the objective to M 1 . 
M 1 serves to reflect the rays from the objective to the screen. 

V Ventilator. M 3 and M 4 are mirrors for use in reflecting the light through 
horizontal transparent objects. 



CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 177 

This apparatus is designed to project opaque objects as large as 22 centi- 
meters in diameter, at a magnification of five to ten with a 30 ampere current. 
For a smaller object one may magnify as high as 25 diameters. With a 50 
ampere current and a larger reflector the magnification may be from 14 up to 
37 diameters. 

In this instrument the carbons are horizontal and in the optic axis. The 
parabolic reflector (R) serves to direct the light in a parallel beam along the 
line of the optic axis. 

It takes considerable time for the eyes to adjust themselves, 
hence, if one passes quickly from opaque projection to lantern 
slides the screen images are dazzling. On the other hand in passing 
from lantern-slide images to opaque images, the eyes being adjusted 
for daylight vision, the screen images seem exceedingly dim at 
first, although the screen image may be as brilliant as it is possible 
to obtain with the best apparatus. After the eyes gain their 
twilight vision the images on the screen appear much brighter, as 
if the light had been greatly increased. As old observers put it: 
"It is necessary to get the brilliant sunshine out of the eyes before 
the relatively dim screen images are satisfactory." 

282. Dim and brilliant light in combined projection. This 
difficulty can be avoided in two ways : 

1. In showing lantern slides, the current may be lessened until 

the light forming the image of the transparency is of about 
the same intensity as is that of the opaque object with the 
full current. 

2. A neutral tinted glass of the proper shade can be put in the 

path of the beam going to the lantern slide, to tone down 
the brilliancy ( 282a). 



282a. In 1908-1909 this difficulty was in part overcome by Mr. A. O. 
Potter by putting a tinted glass of the proper light reducing power in the path 
of the beam going to the lantern slide. This reduces the image of the trans- 
parency to the same dimness as the opaque object, hence one can pass from 
one to the other without any adjustment of the eyes. 

If only lantern slides are to be shown, the tinted glass can be removed and 
the full light employed. 

Some combined lanterns, as those of the Bausch & Lomb Optical Co., and 
perhaps others, are now regularly supplied with the light reducing glass for the 
magic lantern part. 



178 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII 

; ' ^ I V' *fct 




FIG. 96. UNIVERSAL PROJECTION APPARATUS WITH THE PROJECTION 
MICROSCOPE IN POSITION. 
(Cut loaned by E. Leitz). 



CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 179 

This apparatus is designed for all kinds of projection, and with the objects 
either in a vertical or in a horizontal position. When the object is in a vertical 
position the illuminating device (arc lamp with parabolic reflector) sends the 
light horizontally through the specimen, apparatus and to the screen as would 
be the case in the figure here shown. 

If the object is in a horizontal position the lamp and reflector remain in a 
horizontal position and the light is reflected by a mirror upon the opaque 
object; or for vertical opaque objects the radiant is turned sidewise. 

For transparencies in a horizontal position the lamp and reflector are lowered 
to the level of one of the mirrors below, and this mirror reflects the horizontal 
beam up through the transparent object whence it passes to the projector and 
the screen. 

The entire apparatus is covered by a dark curtain (compare fig. 95). 



USE OF OPAQUE PROJECTION FOR EXHIBITIONS AND FOR 
DEMONSTRATIONS 

283. Testing the lantern. The directions given in Chapter I, 
26 are applicable here. 

284. Size of objects for opaque projection. The size of 
object which can be shown with an opaque projector varies greatly. 
The smallest size is usually larger than a lantern slide. The lan- 
tern-slide opening is rarely greater than 6.5 x 7.5 cm. (2.6 x 3 in.), 
while the smallest picture usually shown in the opaque lantern is 
rarely less than postal card size (8 x 12.5 cm., 3x5 in.). From 
this minimum the size ranges all the way up to 50 cm. (20 in.) 
square. 

Of course the radiant and condenser must vary accordingly 
(see fig. 107). 

285. Objects for opaque projection. The best of all are dull 
white objects, like marble figures, or black print on white paper, 
pictures in black and white. Colored pictures in which the bright 
colors of the spectrum like red, yellow and green, are predominant, 
give good images. Metallic objects with polished surfaces give 
good images. Among these the works of a watch or small clock 
show well; also coins and medals. Bright metallic objects show 
best on a dark ground. 

Objects and pictures which are very light-absorbing naturally 
will not give good screen images, no matter how brilliant the light 
or good the apparatus. If the outlines of such objects are what is 



i8o PROJECTION OF IMAGES OF OPAQUE OBJECTS ICn. VII 




FIG. 97. THOMPSON'S REFLECTOSCOPE, MODEL G-2, 1913. 
(Cut loaned by A. T. Thompson & Co.). 

As here shown the instrument is ready for opaque and for transparency 
projection. 

There are additional attachments by which microscopic projection can be 
done with either a horizontal or a vertical microscope. There is also an 
arrangement for placing the magic lantern objective in a vertical position, 
and thus projecting horizontal objects. 

Commencing at the right : The lamp-house with arc lamp and condenser. 
This is at an angle so that opaque objects in a vertical position are lighted 
directly as in Chadburn's opaque lantern (fig. 92). In this case the screen 
picture has the rights and lefts reversed. 

Above is the magic lantern objective for transparencies. 

Below is the large aperture, long focus projection objective for opaque ob- 
jects. The objective is inserted in the dark chamber containing mirrors for 
reflecting the light upward for transparency projection, or downward for the 
opaque objects in a horizontal position. 

Above is shown a lantern slide in the carrier and below a book in a horizontal 
and a picture in a vertical position. 

With the opaqt:e object in a horizontal position the light is reflected from a 
mirror down upon the object, the light from the opaque object is then reflected, 
in part, back to the same mirror and from the mirror out through the projec- 
tion objective to the screen. The screen image in this case will be erect in 
every way if properly placed on the holder. 



CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 181 

wanted very good results can be obtained by using a white back- 
ground. They will appear like silhouettes, but almost no details 
will show. 

286. Screens for opaque projection. On the whole no screen 
is so satisfactory as a white one of the best quality (see 621). 

If the room is narrow, so that all the spectators are included 
in about 30 degrees, the metallic screen answers fairly well. If the 
room is wide, those on the sides near the screen will get only a very 
dim screen image from the metallic screen. With the white screen 
it is practically as good in one place as in another, for the reflection 
is about equal throughout the entire 180 degrees ( 622, 630). 

For darkening the room see 280 and 608. 

287. Magnification of the picture and size of screen image. 

For lantern slides the magnification can be 30 to 60, with resulting 
brilliant pictures; but with opaque projection one can rarely 
magnify more than six to ten times and get good results. 

If the area to be shown is relatively small and the illuminating 
beam is made converging and a powerful radiant (50 amperes) is 
used, the magnification may be carried up to 25 or 37 diameters 
(Zeiss, p. 6) or perhaps more. 

The screen image should not exceed 2 x 2, or 3 x 3 meters (8 x 10 
feet), (Zeiss, p. 6). 

288. Screen distance. In opaque projection, the screen 
images are usually not magnified so much as lantern-slide images 
and the screen distance is usually from three to ten meters. The 
correct magnification (six to ten) is obtained by using an objective 
of the proper focal length, i. e., for a magnification of six and a 
screen distance of three meters there should be an objective of 50 
cm. or 20 in. If the magnification is to be 10 and the screen dis- 
tance three meters then the objective should have a focus of 30 cm. 
or 12 inches. For the discussion relating to magnification, screen 
distance, and focus of the objective see 3Q2a. 

Sometimes it is necessary to project at a screen distance of 1 5 to 
20 meters (50 to 70 feet) . As the magnification of the screen image 
must not usually exceed six to ten, a very long focus projection 



182 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII 




FIG. 98. THE NEW REFLECTING LANTERN OF WILLIAMS BROWN & EARLE 
(No. 3 BR 15). 

(Cut loaned by Williams Brown & Earle). 

This is a combination projector for lantern slides and for opaque objects. 

Commencing at the right: 

N Arc lamp in the lamp-house with the feeding screws extending beyond 
the lamp-house. 

M Lamp-house of metal with the ventilator at the top. 

C First element of the condenser for giving approximately parallel rays. 

D The opaque object in position. The light from the lamp shines directly 
upon it and is reflected outward toward the projection objective (). 

E Projection objective for opaque objects. 

F Mirror for reflecting the image of the opaque object to the screen and for 
correcting the right to left inversion. 

B Water-cell and second element of the condenser for transparency pro- 
jection. 

A Opening for the lantern-slide carrier. 

L Projection objective for lantern slides. 

For lantern-slide projection a mirror at C is brought into position to reflect 
the light out along the optic axis of B and L. 

objective must be used for such a screen distance. (For a magnifi- 
cation of six and a 15 meter screen distance, an objective of 250 cm. 
(100 inches) is necessary). 

289. Arc lamp and amount of current. If one wishes to use 

more than 25 amperes, the arc lamp should be hand-feed. Up to 
25 amperes, the right-angled carbons work well. Beyond that 
amount the inclined or vertical carbons are more satisfactory for 



CH.VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 183 

the right-angled arc goes out easily from the magnetic blow when 
the current is above 25 to 30 amperes. Of course, in opaque pro- 
jection, where the most powerful light available is demanded, 
alternating current is far less satisfactory than direct current ; still 
with skillful application of the light available even alternating 




FIG. 99. THE INDEPENDENCE POST-CARD PROJECTOR. 
(Cut loaned by Williams Brown & Earle). 

This is in principle exactly like Chadburn's opaque lantern with two lamps 
(fig- 93)- I n this projector the lamps are usually of the incandescent form, and 
connection is made with the house-electric lighting system. 

current radiants give fairly good opaque projection (see Ch. XIII, 
7 53 a for size of carbons with different currents, etc.). 

For favorable objects and good conditions one must use not less 
than 20 to 25 amperes of direct current for successful screen pic- 
tures of opaque objects. Those with most experience in the work 
use 40 to 50 amperes. 

For alternating current satisfactory results can hardly be 
obtained with less than 40 amperes, and 60 to 80 are better. 



1 84 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Ca. VII 




FIG. 100. HOME BALOPTICON FOR OPAQUE OBJECTS. 

(Cut loaned, by the Bausch & Lomb Optical Co.). 

In this instrument there is used a small arc light for attachment to the house 
lighting system. The rheostat is shown at the left. 

The object is horizontal and the lamp shines in part directly upon the object 
and in part the light is reflected upon the object by a mirror. From the object 
light is reflected to a mirror above the arc light, and from the mirror directed 
out through the objective to the screen. The projected mirror image appears 
erect on the screen. 




FIG. 101. HOME BALOPTICON FOR LANTERN SLIDES AND 
OPAQUE OBJECTS. 

(Cut loaned by the Bausch & Lomb Optical Co.). 

opaque projection is precisely as in fig. 100. For lantern-slide projec- 
tion the mirror in front of the arc lamp is turned up out of the way and the 
light passes on to the condenser, lantern slide and objective as in ordinary 
lantern-slide projection (fig. i). 



CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 185 




FIG. 1 02. UNIVERSAL PROJECTOSCOPK. 
(Cut loaned by C. II. Stoelting Company). 

This instrument as shown in the picture is designed to project: 

(1) Lantern slides and other transparencies in the usual vertical position 
or in a horizontal position. 

(2) Opaque objects. 

(3) Microscopic objects. For this the lantern-slide objective is turned 
back and the microscope turned up in place. 



1 86 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII 

290. Precaution for heavy currents. The lamps for heavy 
currents are mostly of the hand-feed type and burn large carbons. 
When starting the lamp it is much safer to make sure that the car- 
bons are separated before closing the knife switch. Then one can 
use the feeding screws and bring the carbons together to strike the 
arc, and separate them a short distance immediately. If the 




FIG. 103. DIAGRAM OF THE PARTS AND COURSE OF THE RAYS IN THE 

UNIVERSAL PROJECTOSCOPE FOR OPAQUE AND LANTERN-SLIDE 

PROJECTION. 

(Cut loaned by the C. II. Sloelting Company). 

The instrument is here arranged for the projection of opaque objects. The 
mirror, Af,, reflects the parallel beam from the first element of the condenser 
(C), down on the horizontally placed object. The large aperture projection 
objective directly above, and the 45 mirror beyond, project the image upon 
the screen. 

Ordinary lantern-slide projection is shown by the broken lines, (for a de- 
tailed description of all the parts see fig. 16). 



CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 187 

carbons are in contact after striking the arc, so much current flows 
that there is danger of blowing the fuses or burning out some con- 
nection. Be sure that the fuses and wiring are adapted to the 
current (fig. 3, 691). 

291 . Illuminating the entire opaque object. For illuminating 
opaque objects, Zeiss uses the principle of the search-light. That 
is, the tw r o carbons are horizontal, the positive one has its crater 
facing the concave mirror (fig. 95, 96). This mirror then reflects 
the light toward the object. Depending upon its position, it can 




FIG. 104. NEW MODEL CONVERTIBLE BALOPTICON IN POSITION FOR 

OPAQUE PROJECTION. 
(Cut loaned by the Bausch & Lomb Optical Co.). 

In the new (1913) models of projectors by the Bausch & Lomb Optical Com- 
pany provision is made in each case to place the object in a horizontal position 
and then to illuminate it either by a mirror (fig. iO5a) or preferably by tilting 
the radiant and first element of the condenser (fig. 105), so that the light from 
the lamp is projected directly upon the object. From the object a part of the 
light extends out through the vertically placed projection objective to the 
mirror and from the mirror to the screen. The mirror gives correct images on 
the screen. 



1 88 PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. VII 

direct a parallel beam, a converging or a diverging beam (see also 
Ch. XIII-XIV on radiants and lighting). 

If a condenser is used, its size must be adapted to the size of the 
object, that is, the diameter of the cylinder of light must be some- 




FIG. 105. DIAGRAM SHOWING THE OPTICAL PARTS AND THE COURSE OF 
THE RAYS IN THE CONVERTIBLE BALOPTICON IN OPAQUE PROJECTION. 

(Cut loaned by the Bausch & Lomb Optical Co.). 

The lamp-house, radiant and first element of the condenser are so inclined 
upward that the light from the condenser falls directly upon the opaque object. 

A Upper carbon of the arc lamp furnishing the light. 

B First element of the condenser to render the diverging light parallel. 
The lens beyond the meniscus is double-convex instead of plano-convex as in 
fig. 3- 

D Position of the opaque object. Objects as large as 20 x 20 cm. (8x8 
inches) can be illuminated and projected. 

E Large aperture projection objective in a vertical position. 

F Mirror beyond the objective to reflect the image to the screen and correct 
the inversion. 

C Mirror. It serves to increase the illumination of the opaque object by 
reflecting back upon it some of the scattered light. 

6" Second element of the condenser for lantern-slide projection (fig. 3). 

// Projection objective for lantern slides. 

Bellows. 

M Lathe bed on which slide the objective, etc. 



CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS i! 




FIG. losa. DIAGRAM SHOWING THE COURSE OF THE LIGHT RAYS FOR 
TRANSPARENCY AND OPAQUE PROJECTION WITH THE RADIANT HORIZONTAL. 

(Cut loaned by the Bausch Of Lomb Optical Co.). 

A Upper carbon of the arc lamp. 
B The first element of the condenser (fig. 3). 

C C Mirror horizontal when using lantern slides and inclined for opaque 
projection. 

D Horizontal surface for opaque objects (20 x 20 cm., 8x8 in.). 

E Projection objective for opaque objects. 

F Mirror for reflecting the light to the screen and correcting the inversion. 

G Second element of the condenser for lantern slides. 

// Projection objective for lantern slides. 

N Support for condenser and bellows. 

Bellows. 

M Lathe bed on which move the various supports. 

what greater than the diagonal measuring the size of the picture, as 
for lantern slides (see 314, fig. 114). A diverging beam could be 
used by pushing the radiant within the focal distance, and a con- 
verging by separating farther than the focal distance. Sometimes 
there is no condenser but the radiant shines directly upon the 
object (fig. 99, 100, 107). 



190 PROJECTION OF IMAGES OF OPAQUE OBJECTS [CH. VII 

292. Avoidance of shadows. With solid objects there will be 
very heavy shadows unless the light is evenly distributed. With a 
single lamp this is not easily accomplished, and if no mirror is used 
practically impossible. It is better to use two lamps, one on each 
side, as in the original apparatus of Chadburn (fig. 93). The two 
lamps have the further advantage of doubling the light. Two arc 
lamps are used in the large opaque lantern of the Bausch & Lomb 
Opt. Co. (fig. 107). 

In the Spencer Lens Co.'s opaque lantern, plane mirrors line a 
part of the projection chamber where the object is placed, and much 
of the light lost by absorption without this arrangement is reflected 
back upon the object. This also helps to obviate the shadows 
when one lamp is used (fig. 1 1 1). 

ERECT IMAGES WITH OPAQUE OBJECTS 

293. Inversion of the image with an opaque object. Besides 
being upside down the image of an opaque object on an ordinary 
white screen has the rights and lefts reversed. 

294. How to get an erect image with the object in a vertical 
position. Put the opaque object in the vertical position upside 
down. Point the objective at right angles to the screen, use a 
mirror at 45 degrees, or use a 45 degree prism to direct the image- 
forming rays upon the vertical opaque screen (fig. 95, in). If 
the inversion of the rights and lefts is unimportant, put the object 
upside down in the vertical holder and point the objective directly 
toward the screen (fig. 97, 109). 

If a translucent screen like ground glass is used the image will be 
erect in every way if it is put upside down in the holder and the 
objective pointed directly toward the screen. 

295. How to get an erect image of an opaque object in a 
horizontal position. Place the opaque object with its upper edge 
away from the screen. The objective is usually in a vertical 
position so that the image would appear on the ceiling above the 
instrument. The mirror or prism used to direct the image forming 
rays upon the vertical screen corrects also the mirror image, and 



CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 191 




FIG. 106. NEW MODEL UNIVERSAL BALOPTICON IN POSITION FOR 
OPAQUE PROJECTION. 

(Cut loaned by the Bausch & Lomb Optical Co.). 

Opaque objects are placed in a horizontal position and the lamp-house, lamp 
and first element of the condenser are inclined as in fig. 105. The light from 
the opaque object is reflected upward to the right face of an inclined mirror 
and from the mirror reflected out through the projection objective, giving an 
erect screen image. 

When used for lantern slides the lamp-house is horizontal and the horizontal 
light is reflected upward by the left face of the mirror to the mirror at the left 
of the lantern-slide attachment. This second mirror reflects the light hori- 
zontally through the lantern slide. 

the object will be erect in every way (fig. 95-111). (See also the 
discussion of the reflecting lantern of Thompson in which a mirror 
image is projected, and hence appears erect on the screen (fig. 97, 
100). If a translucent screen is used with the object in a hori- 



1 92 PROJECTION OF IMAGES OF OPAQUE OBJECTS lCn. VII 




FIG. 107. BAI.OPTICON FOR THE PROJECTION OF LARGE OPAQUE OBJECTS. 

(Cut loaned by the Bausch & Lamb Optical Co.). 

This opaque projector is especially designed to show large objects and large 
surfaces (20 inches, 50 cm. square). To avoid shadows in projecting machines 
and other solid objects, and to supply the needed illumination there are two 
25 ampere lamps tilted to throw their light directly upon the two opposite 
sides of the object. Each lamp has its own rheostat and table switch. 



CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 193 

The projection objective is of the Tessar Ic series of very large aperture 
(114 mm., 4T3 in., in diameter and 50 cm. 19^ in. equivalent focus). The 
focusing is accomplished by a screw which raises or lowers the table supporting 
the object. 

This instrument enables one to demonstrate to an audience the workings of 
a machine like a cash register, or a quarto size page of illustrations or print. 
With the vertical objective and a mirror to reflect the light to the screen the 
image will be erect. The reflecting mirror is silvered on the front to avoid 
the doubling of the image. 




FIG. 108. MODEL 5 DELINEASCOPE FOR OPAQUE AND LANTERN-SLIDE 

PROJECTION. 
(Cut loaned by the Spencer Lens Co.), 

With the arc lamp and the first element of the condenser in a horizortal 
position the light extends directly to the right through the lantern slide or 
other object and the projection objective, or projection microscope, or it ir.ay 
be reflected upward through the vertical projection microscope (fig. 175). 

For opaque projection, the arc lamp and first element of the condenser are, 
by means of the crank, rotated within the lamp-house to the right position to 
direct the light upon an opaque object in a vertical or in a horizontal position 
as desired. 

If the object is in a horizontal position the light from it is reflected to a mirror 
and from the mirror out through the large projection objective. It will appear 
correct in the screen image. The vertical object will have the rights and lefts 
inverted. Objects or surfaces 15x23 cm. (6x9 in.) can be projected with 
this instrument. 



1 94 PROJECTION OF IMAGES OF OPAQUE OBJECTS [CH. VII 

zontal position the image will only be erect with the screen at right 
angles to the axis of the objective, no mirror or prism being used. 
If a mirror or prism is used to project upon a vertical screen then a 
translucent screen will give a mirror image, but an opaque screen 
an erect image. 




FIG. 109. DIAGRAM FHOWING THE PARTS AND COURSE OF THE RAYS IN* 

MODEL 4-5 DELINEASCOPE. 
(Cut loaned by the Spencer Lens Co.). 

This diagram shows the arc lamp and first element of the condenser in posi- 
tion to illuminate a vertical object in opaque projection. Above is shown in 
outline the course of the rays for the projection microscope or the magic lantern. 

Commencing at the left : 

OH The object holder for objects 15 x 23 cm., 6x9 in. 

II Handle for operating the object holder. 

X The horizontal axis on which rotates the arc lamp and the first element 
of the condenser. 

OO The large projection objective for opaque objects. 

WC Water-cell for removing the radiant heat. 

ID Large iris diaphragm. 

L Lantern slide, and crank for turning the slide up in the vertical position 
in front of the condenser behind the objective. 

P Platform on which is laid the lantern slide. 

LO Lantern-slide objective turned to one side to allow the microscope to 
get to the horizontal position. 

M Mirror to reflect the horizontal beam of light through the vertical 
microscope. 



CH. VII] TROUBLES IN OPAQUE PROJECTION 195 

296. Troubles: 

i . The one great trouble will be a dim screen image. This can- 
not be wholly avoided. It can be made tolerably good: 
( i ) by having the room very dark ; ( 2 ) by using a powerful 
radiant; (3) by having a projection objective of large 
aperture; (4) by magnifying the screen image very 
moderately (5 to 10 diameters). 




FIG. no. MODEL 8 DELINEASCOPE FOR ALL KINDS OF PROJECTION. 
(Cut loaned by the Spencer Lens Co.). 

In this instrument there is provision for lantern-slide projection with the 
slides or other objeets in a vertical or in a horizontal position. 

It provides for opaque objects in a horizontal position and lighted directly 
by the radiant (fig. in), and for objects in museum jars in a vertical or hori- 
zontal position. 

Finally it provides for micro-projection with the objects in a vertical position 
or in a horizontal position, and for the drawing of objects on a horizontal or on 
a vertical surface. 



TROUBLES IN OPAQUE PROJECTION 



[CH. VII 




FIG. in. DIAGRAM OF MODEL 8 DELIKEASCOPE SHOWING THE POSITION 
OF THE RADIANT AND THE COURSE OF THE LIGHT RAYS FOR OPAQUE 
PROJECTION WITH THE OBJECT IN A HORIZONTAL POSITION. 
(Cut loaned by the Spencer Lens Co.). 

T Table for opaque objects. 

W Wheel by which the table is raised and lowered. 

D Diaphragm above the table for flattening out the page of a book. 

B Incandescent bulb which always gives light for the interior of the 
machine. 

C Condensing lenses in front of the arc. 

O Large objective for opaque projection. 

() 1 Smaller objective for vertical projection. 

,]/ Mirror for throwing light downward for the lantern-slide compartment 
or upward through the vertical attachment. 

-If, Mirror for reflecting a perpendicular beam of light out through the 
lantern-slide compartment; shown thrown up against the water-cell in this 
figure (see fig. 177) 

M 3 Mirror used in connection with projection of the vertical side of an 
object. 

M^ Mirror which assumes a position at 45 when the microscope is used 
perpendicularly. 

P Prism which is thrown into the prism box when the microscope is used 
in a perpendicular position. 

S Shelf upon which the lantern slide is placed previous to throwing it up 
into the optical axis by the handle. 

// Handle of the lever for raising the slide into position. 



CH. VII] TROUBLES IN OPAQUE PROJECTION 197 

2. If the amperage is to exceed 25 or 30, it is better to use an arc 

lamp with inclined or vertical carbons, not those at right 
angles for the magnetic blow puts the right-angled arc out 
too easily. 

3. Do not have the carbons in contact with a hand-feed lamp 

when the current is turned on. Feed them together after 
the current is on, then they can be separated properly 
immediately after the arc is struck. 

4. Inverted screen image. The object not properly placed on 
{ the support, or no erecting mirror or prism is used. 

5. No detail in the screen image. The object may be too light- 

absorbing, or the light may not be sufficient. 
(See Troubles in Ch. I.). 



DO AND DO NOT IN OPAQUE PROJECTION tCn. VII 



297. Summary of Chapter VII: 



Do 

1. Select an objective of large 
aperture for opaque projection 

( 275)- 

2. Use a light of great brill- 
iance like sunlight or the arc 
light ( 274, 277). 



3. Make the screen image 
only six to ten times as large as 
the object ( 287). 

4. Make the projection room 
very dark ( 280). 

5 . Use a very white screen or 
under some conditions a metal- 
lic screen ( 286, 621). 

6. From 25 to 50 amperes of 
direct current are needed to 
give good opaque projection 

( 289). 

7. If lantern slides and 
opaque objects are projected at 
the same exhibition, use a 
neutral tint (smoky) glass to 
make the lantern-slide image as 
dim as the image of the opaque 
object ( 282). 

8. Use a condenser for opaque 
objects somewhat larger than 
the object (see fig. 114). 



Do NOT 

1. Do not undertake opaque 
projection with an objective of 
small aperture. 

2. Do not expect good 
opaque projection unless from 
20 to 50 amperes of direct cur- 
rent, or greater amperages of 
alternating current are avail- 
able. 

3. Do not try to magnify the 
object too much. 

4. Do not try to project in a 
light room. It must be dark. 

5. Do not be satisfied with a 
dirty, non-reflecting screen. It 
must be white. 

6. Do not expect brilliant 
screen images with a weak light. 



7. Do not pass quickly from 
the dim pictures of opaque 
objects to the brilliant pictures 
of transparencies. Dim the 
transparencies down to the 
opaque images. 



8. Do not use a small con- 
denser for a large object. 



CH. VII] DO AND DO NOT IN OPAQUE PROJECTION 



199 



9. Use two radiants or mir- 
rors for avoiding shadows with 
solid objects ( 292). 



10. Select objects which re- 
flect well for opaque projection 
( 285). 

11. If very light-absorbing 
objects must be projected, use a 
white background ( 285). 

1 2 . Use a hand-feed arc lamp 
for opaque projection ( 289, 
290). 

13. Make sure that the wir- 
ing is adapted to the heavy 
currents needed for opaque pro- 
jection ( 290). 

14. Use carbons of the proper 
size for the current drawn 

( 290, 7S3a). 

1 5 . Make the images erect by 
placing the object up-side down 
for the vertical position, or with 
the upper edge away from the 
screen for the horizontally 
placed objects ( 293-294). 

1 6. Use a mirror or prism to 
avoid a mirror image on a ver- 
tical, opaque screen ( 293- 
295)- 



9. Do not light solid objects 
so that there will be deep 
shadows. Use two radiants, or 
mirrors, or arrange so that the 
light strikes the object directly, 
not obliquely. 

10. Do not select badly re- 
flecting objects for opaque pro- 
jection. 

1 1 . Do not use a black back- 
ground on which to place dark 
objects. 

12. Do not use an automatic 
right-angle carbon arc lamp for 
the heavy currents needed for 
opaque projection. 

13. Do not nm any risks by 
using the heavy currents on 
wiring not adapted to it. 

14. Do not use small carbons 
for big currents. 

15. Do not get the images 
wrong side up on the screen. 



1 6. And do not expect too 
much in opaque projection. 
Know the principles involved; 
study fig. 90-91. 



CHAPTER VIII 
PREPARATION OF LANTERN SLIDES 

310. Apparatus and Material for Chapter VIII: 

A photographic dark room; Camera with suitable objectives and 
plate holders (fig. 116-119); Lantern-slide plates, negative plates 
of various kinds; Chemicals for developing, etc.; Colors and 
brushes for tinting the slides; A retouching frame (fig. 113); 
Cover-glasses and binding strips and mats for the slides ; Markers 
and labels for the slides; Cabinet for the slides (fig. 120). 

311. For the historical development of lantern slides see the 
works referred to in Ch. I, 2, and for photographic lantern slides, 
The Journal of the Royal Society of Arts, Vol. LIX (191 1), pp. 

255-257- 

For making and coloring lantern slides see the works inCh. I, 2, 
and Lambert, Lantern-slide making and coloring. 

The Photo-Mineature series No. 9, Lantern Slides, and No. 83, 
Coloring Lantern Slides. 



312. Modern lantern slides are of several standard sizes as 
follows: (See3i2a). 

A. American slides. These are oblong plates 82.5 x 102 mm. 
(3^ x 4 inches). They are designed to go into the lantern -slide 
carrier with the long side horizontal ( 35). 

B. British slides. These are square, being 82.5 x 82.5 mm. 
(3 Y* x Z% inches) (37). 

C. French slides. These are, following the recommendations 
of the French Congress of Photography for 1889, 85x100 mm. 
(3 n /32 x 3 lr ri6 inches). That is, the standard is practically like the 
American, and French slides can be used in American lantern-slide 
carriers. 

D. German slides. In Germanic countries, slides of 85 x 100 
mm. are much used, but the German standard is often given as 
90 x 120 mm. (3%; x \y inches). Those of 130 x 180 mm. are 
likewise employed. 



CH. VIII] 



PREPARATION OF LANTERN SLIDES 



201 



E. Italian slides. In Italy the sizes are 85 x 85 mm., 85 x 100 
mm. and 90 x 120 mm., that is, the British (B), the French and 
American (A, C) and German (D) sizes. 

In all countries those of larger and smaller sizes than the above 
standards are used for special purposes; and provision is made 




FIG. 112. AN AMERICAN LANTERN SLIDE, FULL SIZE, WITH INSTRUCTIONS 

FOR MAKING LANTERN SLIDES DIRECT. THE SLIDE is PROPERLY 

"SPOTTED." 

everywhere for the square British slides of82_5x82.5 mm. and also 
for the oblong form of 82 or 85 x 100 mm. of the French and Ameri- 
can manufacturers. 

Any oblong form has the advantage that it is always put into 
the carrier with its long side horizontal and therefore requires only 
one mark or spot to indicate how it shall be inserted for an erect 



202 PREPARATION OF LANTERN SLIDES [Cn. VIII 

image (fig. 6-8, 112). For a square form two marks are needed 
(fig. 13, 113). 

3 13. Actual size of the free opening with lantern slides. The 

sizes given above are the measurements from the extreme edges 
of the plates. The actual size of the picture to be projected is 
always less, as part of the slide is covered when inserted in the 
carrier. The mat between the slide and its cover, and the binding 
around the edge lessen the size a variable amount. It requires 
from 5 to 10 mm. all around the edge for the binding and the part 
covered by the slide-carrier. This leaves a clear opening in the 
lantern slide of that much less. The smaller the slide to start with 
the less will be the proportionate amount of clear space left after 
the mounting of the slide. 

The free opening of the American slides is rarely greater than 
70 x 75 mm. and much more frequently the free opening is con- 
siderably less. 

314. Diameter of the condenser required for different sized 
lantern slides. The final element of the condenser next the lantern 
slide (fig. i, 2, 114) must be somewhat greater in diameter than 
the diagonal of the free opening of the lantern slide to be projected. 

The accompanying figures show the British, French and Ameri- 
can, and German standard sizes of lantern slides with the minimum 
diameter of the condenser which should be used with them (fig. 
114). 

312a. There is some confusion as to the exact outside measurement of 
lantern slides. For example, the exact size of the British square slides is 
3>4*3/4 inches (82.5x82.5 mm.) In the two French works consulted 
(Trutat, p. 311, and Fourtier, tomeii, p. 18) the British size is given as 80 x 80 
mm. 

In Italy the size is given as 85 x 85 mm. In the German work of Wimmer 
the exact size is given (82.5 x 82.5 mm.). Neuhauss speaks of slides 85 x 85 
mm. (p. 27). 

The standard French slides are given as 85 x 100 mm. This is one of the 
standard sizes in Germany and Italy. Hence, it is concluded that the standard 
British slide is meant whenever 80 x 80, 82.5 x 82.5, or 85. x 85. mm. slides are 
mentioned. Also that the standard French and American slide of 3^x4 
inches (82.5 x 100 mm.) is meant whenever slides of 85 x 100 mm. are men- 
tioned. 



CH. VIII] 



PREPARATION OF LANTERN SLIDES 



203 



315. Making lantern slides. In the use of the lantern at the 
present day, one will find occasion to make lantern slides by all of 
the different ways that have ever been devised. That is, they may 
be drawn or painted wholly by hand ; made partly by photography 




FIG. 113. BRITISH LANTERN SLIDE OF FULL SIZE WITH TWO "SPOTS." 

The "spots" are on the upper corners in the English slides. 

The picture shown on the slide is of a retouching stand suitable for use in 
coloring slides. 

5 The slide. 

R A reflector to throw the light up through the slide. This may be a 
mirror or simply white paper. 

and then hand-colored; made wholly by photography, or trans- 
parent natural objects may be used. 

Natural objects of the right transparency may be mounted on 
glass slides and used in the lantern. For example, seaweeds, thin 
leaves, skeletonized leaves, large wings of insects; crystals on 



2O4 



PREPARATION OF LANTERN SLIDES [Cn. VIII 



glass, thin sections of wood or animal organs mounted on glass, 
fibers of wood, thin cloth, spiders' webs, etc., etc. 




FIG. 114. STANDARD BRITISH, FRENCH, AMERICAN AND GERMAN LANTERN- 
SLIDES WITH THE CONDENSER NECESSARY TO FULLY ILLUMINATE 
THEM. (ABOUT HALF NATURAL SIZE). 

316. Hand-made lantern slides. Practically no one now 
makes the beautiful hand-painted lantern slides of former times; 
but for outline diagrams, for tables and for short statements, it is 
easier and cheaper to make the slides direct than to first make a 



CH. VIII] PREPARATION OF LANTERN SLIDES 205 

diagram or table, etc., and then have a photographic lantern slide 
made. 

In preparing these slides direct, a device of the artists of earlier 
times who painted lantern slides, is used. That is, the slide is 
cleaned carefully and then coated with a thin solution of some hard 
varnish or with gelatin (fig. 112, 317). After the varnish has 
thoroughly dried one can use a pen or a brush upon the varnished 
surface with the same facility as upon paper. The hand-made slide 
is then mounted as usual and can, of course, be used indefinitely. 

If they are for a special occasion as in projecting election 
returns, games, etc., the slides are used without a cover-glass. 
They may be easily cleaned off with turpentine or xylene and used 
over and over. 

317. Coating the lantern-slide glass with varnish. One of 

the best varnishes for this purpose is composed of 5% dry Canada 
balsam or gum dammar in xylene or in turpentine; or 10% natural 
Canada balsam in xylene or toluene. Or one can take some good, 
varnish, especially Valspar, one part and xylene, toluene, gasoline 
or turpentine nine parts. All of these thin solutions should be 
allowed to stand until they are clear, and only the clear part used. 
If one is in haste it is possible to filter the thin varnish through filter 
paper. 

For coating the glass, the best way is to hold the clean glass flat 
by grasping the edges with the thumb and fingers. Then varnish 
is poured on, and the glass tilted slightly until the whole surface 
is covered. The excess is poured off one corner back into the 
bottle. Then the glass is stood on edge to dry. In a warm dry 
room 15-20 minutes will suffice for varnish in xylene or toluene. 
If turpentine is used it may require half a day or more. When the 
varnish is once dry the glass can be used at any time. 

As it is not easy to tell which side has been varnished, a slight 
mark in one corner of the varnished surface with a glass pencil or 
pen will enable one to tell quickly and with certainty. 

318. Coating the lantern-slide glass with 10% gelatin. For 

this, some clear gelatin is made into a 10% solution in hot water, 



206 PREPARATION OF LANTERN SLIDES [CH. VIII 

and filtered through filter paper. The slides are coated with the 
gelatin as described for the varnish. When the gelatin is dry the 
surface receives a pen or brush well. Gelatin slides are not so 
satisfactory as the varnished slides. 




FIG. 115. AMERICAN LANTERN SLIDE OF FULL SIZE WITH GUIDE LINES 
FOR MAKING SLIDES DIRECT. 

The thumb tacks at the four corners are to hold the slide firmly in position 
while writing or drawing upon it. The lined area represents about the maxi- 
mum size of opening projected in ordinary work (65 x 75 mm.), (2>i X2 %in.). 

319. Inks and pens. One can use any ink and any pen on 
the varnished or gelatinized slides. 

For making tables, etc., it is best to use water-proof India ink 
and a fine pen, a crow-quill, steel pen is excellent. 

320. Drawing diagrams on varnished slides. One can draw 
freehand on these varnished slides as well as upon paper. For 



CH. VIII] PREPARATION OF LANTERN SLIDES 207 

those not especially skillful, it is probably better to draw the sketch 
first and then trace the sketch on glass as follows: Place the 
lp,ntern-slide glass on the drawing, varnish side up, and arrange it 
as desired. Select very thin glass for this, so that the drawing sur- 
face will be near the picture to be traced. Now with a pen or 
brush trace the outlines. One can also use colored inks if desired. 

321. Guide for table making and for writing. For making 
lantern-slide tables or written matter direct on the slide it is best 
for most workers to have a guide which shall show the maximum 
size which can be projected (fig. 115). If one has no special guide, 
cross-section paper or catalogue cards will serve well. 

To hold the glass in position while writing or making diagrams, 
thumb tacks at the corners are efficient (fig. 115). 

322. Ink and pen to use on unvarnished glass. For tem- 
porary use, as in reporting games, etc., the glass is cleaned and 
then the fingers rubbed over it. Now with a ball-pointed pen one 
can write upon the glass. The lines will be coarse, but that will 
not matter. One can write with an ordinary pen also, but not so 
surely as with a ball-pointed pen ( 322a). 

The ink can be of almost any kind. The black India ink gives 
the sharpest images. 

A special ink called "glassine" has recently been put on the 
market. It is in six colors, white, black, red, green, blue and violet. 
The ink is thick and with it one can write on untreated glass with 
any pen, although a ball-pointed pen is here also an advantage 
( 322b). The ink is easily washed off with water so that the same 
glass slide can be used over and over. 



322a. The writers are indebted to Dr. E. M. Chamot for the suggestion 
to use the ball-pointed pens on the unvarnished glass, also the advantage of 
rubbing the fingers or palm over the cleaned glass to prevent the ink from 
spreading. 

According to Lewis Wright, p. 412, one can write on glass well if the glass is 
licked, and the thin coating of saliva so spread upon the glass is allowed to dry. 
The ink will not spread, and the saliva-coated glass takes the pen well. 

322b. "Glassine announcement slide ink." This ink is made by the 
Thaddeus Davids Co., 127 William St., N. Y., and is supplied in I oz. (30 cc.) 
bottles, the full set of six colors costing $1.00. See the Moving Picture World, 
March, 1914. 



208 PREPARATION OF LANTERN SLIDES ICn. VIII 

323. Smoked glass. For some purposes nothing is better 
than smoked glass slides. On these one can write or draw with a 
sharp point either before or during the exhibition. If one takes the 
precaution to commence writing on the lower edge of the slide and 
on the face looking toward the condenser the writing or diagram 
will appear right side up on the screen (see 3 5 for proper position 
of lantern slides in the holder) . 

Smoked slides must be handled carefully or the surface will be 
spoiled. 

324. Thin sheets of mica or of gelatin. On a sheet of mica, 
of gelatin or of non-inflammable cellulose one can write or draw 
with a pen or brush, using any colored ink. India ink is best for 
outlines and for written words, letters, or numerals. 

As these sheets are very thin it is best to put a slide made upon 
one of them between two glasses, so that the sheet will be held flat 
and be protected. (For other methods of hand-made slides see 
Dolbcar, pp. 29-32). 

PHOTOGRAPHIC LANTERN SLIDES 

325. Nearly all of the lantern slides now used arc made 
wholly or in part by photography. 

Negative. First, there is made a negative of the object to be 
represented in the lantern slide. This negative may be on any 
size of plate, but the picture should be, if convenient, of the proper 
size for a lantern slide. That is, its outside dimensions must not 
exceed 75 x 70 mm. (3 x 2.8 in.). 

This negative should be very sharp and free from defects. Any 
lack of sharpness or any defects will come out with distressing 
prominence when the picture is magnified by the lantern. One 
must then use a good objective in making the picture, or if the 
objective is not particularly good a very small diaphragm is used. 
If it is desired that print shall be read easily by all in the room, the 
lantern slide should not have the letters smaller than six point type 
(see fig. 216 for sizes of type). 



CH. VIII] 



PREPARATION OF LANTERN SLIDES 



209 



326. Printing the lantern slide from the negative. If the 

picture on the negative is of the proper size for a lantern slide, it is 
put into a printing frame exactly as for printing with paper. Then 
in the dark room a lantern-slide plate is put with its sensitive side 
next the negative and arranged so that the picture will be straight 
on.the lantern slide. The cover of the printing frame is put on and 
held in place by the hands or by the springs. The exposure may 
be in diffused daylight, or about 30 cm. from any good artificial 
light (incandescent bulb, Welsbach gas light, kerosene lamp). 



Base 





FIG. i I 6. 



CAMERA FOR MAKING LANTERN SLIDES BY MEANS OF AN 
OBJECTIVE. 



Base The base of the camera resting on the table. 

Objective The photographic objective in the middle segment of the camera. 
The objective is shown as if the enclosing bellows were transparent. 

Front The front of the camera where the negative is placed. 

Reflector A white sheet of paper or cardboard placed on a shelf at 45. 
This reflector serves to illuminate the negative. 

By varying the relative distances of ground glass, objective and negative, 
the lantern slide can be larger or smaller or of the same size as the corresponding 
part of the negative. 

The exposure required varies with the negative, but it is less than 
for most developing papers. 

327. Developing the lantern slide. Any good developer may 
be used, but as a rule the directions given in the box of plates are 
the best to use with that brand of plate. One should develop until 
the picture appears clearly. The temptation is to develop too 
much and thus make the slide too opaque. Black, like printed 
letters, should be opaque in the correct lantern slide, but there 
should be all gradations from that to clear glass in the whites. 



PREPARATION OF LANTERN SLIDES 



[Cn. VIII 



Any one who can make a good negative and a good paper print 
from it can make a good lantern slide. The lantern slide is a 
positive and the lights and shades should appear as in the object 
when one looks through the slide toward the light. These lantern 
slides are small transparencies, and some of them make beautiful 
ornaments when used as transparencies in a window. 

There is more danger of getting the slides too opaque than not 
opaque enough. The beginner should try each lantern slide with 




FIG. 117. COPYING, ENLARGING OR REDUCING CAMERA. 

(From the Catalogue of Anthony & Co.). 

The objective. The bellows have been cut away to show it. 

/ Front of the camera with frames or "kits" for negatives of various sizes. 
For making enlargements with this camera the objective can be placed in the 
front . 

a moderate light in the lantern. If the picture on the screen is 
brilliant and shows all the details with the moderate light, it will, 
of course, give a more brilliant picture with the electric light of 
3000 to 4000 candle-power. If the slide is too opaque, it will not 
come out well with the moderate light and, while the powerful 
electric light may show it fairly well, so much radiation will be 
absorbed and transformed into heat that the slide is liable to break 
if left in the lantern a considerable time. The more transparent 
slides allow the radiant energy to pass through them and naturally 
they are not so greatly heated. 



CH. VIII] 



PREPARATION OF LANTERN SLIDES 



328. Negatives as lantern slides. Many objects appear 
equally well and equally clearly when projected from a negative as 
from a positive or transparency. That is, there will be white lines 
and white letters, etc., on a black background. This was a 
favorite method of illustrating in the older works on physics and 
projection. For examples, look at the pictures in Dolbear's Art of 




FIG. 1 1 8. PHOTOGRAPHIC CAMERA UPON A BASEBOARD HINGED TO A 

TABLE. 
(From The Microscope). 

This is one of the copying, enlarging and reducing cameras. The objective 
may be at the end, in a cone, or in the middle segment. For lantern-slide 
making it is in the middle segment and the negative at the end, the whole 
camera being directed upward toward the sky. 

By reversing the position of the camera, and placing the hinged board in a 
vertical position, objects in liquids and any object in a horizontal position can 
be photographed. 

NOTE. The arrangement shown in fig. 1 18 with a baseboard hinged to the 
table, and with a camera which could be placed pointing upward or downward 
was devised by the senior author in 1878 especially for photographing objects 
in liquids or objects which must remain in an inclined or horizontal position. 
The baseboard carrying the camera can be fixed in any position from the 
horizontal to the vertical. (Proc. Amer. Assoc. Adv. Sc. Vol. XXVIII (1879), 
p. 489; Science, Vol. Ill, p. 443, and Vol. IV, p. 5 (1884). 



212 



PREPARATION OF LANTERN SLIDES 



[Cn. VIII 




Projecting, Deschanel's Physics, etc., 
and fig. 141, 190, 211, 214. 

There is one serious drawback to such 
lantern slides. The background being 
nearly opaque stops the light and other 
radiant energy from the lamp, and the 
great heat developed is 
liable to crack the slides 
(see 18, 845). 




FIG. 1 19. FOLMER & SCHWING'S TILTING CAMERA AND ADJUSTABLE BACK. 

(From the Catalogue of Folmer & Sch-wing. Cut loaned by the Eastman 

Kodak Co.). 

A Tilting camera for making lantern slides or other transparencies with an 
objective, or for photographing objects in a horizontal or inclined position. 

B Adjustable back for the tilting camera. The adjustments are to the 
right or left, up or down and enable one to center accurately any desired part 
of the negative or other object to be photographed. The rotary motion of the 
back enables one to get the lines on the negative or object exactly parallel with 
the edge of the lantern slide. 



CH. VIII] PREPARATION OF LANTERN SLIDES 213 

329. Printing lantern slides by the aid of a camera. Unless 
the negatives from which lantern slides are to be made have the 
part to be shown of exactly the size of a lantern slide, the trans- 
parency or positive cannot be printed by contact. Then one can 
use a photographic camera and print the transparency as follows : 
The negative is put in a suitable opening or in the proper "kit" or 
frame in the end of a copying camera (fig. 116-119), and the 
objective in the second segment. The picture or film side of the 
negative must face the objective. Then the end of the camera 
holding the negative is elevated sufficiently to get a sky background 
through the window ; or the camera is left level and a large piece of 
cardboard or white blotting paper is set at an angle of about 45 
degrees out of a window and the camera pointed toward it. In 
either case the entire lantern slide will be evenly illuminated and a 
good print can be obtained. 

Now focus the picture of the negative sharply on the ground 
glass of the camera and get it of the proper size by pulling out or 
closing up the bellows. 

Print the positive by putting a lantern-slide plate in the plate 
holder in the usual manner and exposing it. Then develop as 
usual. 

It is to be noted that the film surface of the negative and the 
sensitive surface of the lantern-slide plate face each other by this 
method exactly as for contact printing ( 32ga). 

330. Camera for lantern slides. If one is to make many 
lantern slides it is a great convenience to have available a special 



329a. White prints on a black ground. By using an ordinary negative 
giving black lines on a white ground one can get white lines or a white picture 
on a black ground by applying the method just given for printing lantern slides 
by means of a camera and an objective. Place the negative in position, but 
with the film side facing away from, not toward the objective as for an ordinary 
lantern slide. Use a lantern slide or any other kind of plate and make the 
picture just as for the lantern slide. The glass picture thus produced will be a 
positive like a lantern slide but it will have all the parts reversed exactly like a 
negative. If now this picture is used as a negative and printed with cyco, 
velox, argo, haloid or any other printing paper the picture will appear white on 
a dark ground. 

Of course, any lantern slide can be used for making prints, but the picture 
will be reversed in every way, the lights and darks, the printing, etc. To pre- 
vent the inversion of the printing one can use an objective and camera as 
described in Ch. X, 512. 



214 PREPARATION OF LANTERN SLIDES [Cn. VIII 

camera known as a "copying, enlarging, and reducing camera" 
(fig. 116-119). As seen from the picture, the objective is placed 
in the middle segment if lantern slides are to be made from nega- 
tives, and the negative is placed in the proper sized frame or "kit" 
at the end of the camera. No light then reaches the negative 
except on the face looking toward the light, hence there will be no 
trouble from reflections. 

In the best form of these cameras there is a "back with revolving, 
rising and vertical sliding lantern-slide attachment" for printing 
and for making the negatives (fig. 119). The picture can be got 
on the plate in the exact position desired, i. e., lines of print, etc., 
exactly parallel with the edge of the plate. By means of a camera 
one can print lantern slides from the negatives before they are dry. 
This is sometimes a great convenience. 

331. Printing lantern slides by artificial light. With contact 
printing one can use daylight or any convenient artificial light 
petroleum, gas, acetylene or electric. For printing with the 
camera, however, it is not so easy to get the negative evenly 
illuminated. A good way to evenly illuminate the negative is to 
use a 45 degree cardboard reflector illuminated with one or two 
incandescent lights, preferably with frosted bulbs in a horizontal 
position. Mantle gas lights serve well for illuminating the card- 
board. The negative is set vertically some distance from the card- 
board. 

The time for printing lantern slides by contact or by the aid of a 
camera will vary with the negative as for paper prints; much 
depends on the intensity of the light and on the rapidity of the 
plates used. 

To give an example of the time required in a given case the 
following table is added : 

The same objective with a diaphragm opening of F/8 was used 
for all, and the same negative was used in each case. All the plates 
were from the same box and the same developer was used for all, 
so that the only variable was the light. 

1. Sky background, diffused light 10 seconds. 

2. Cardboard at 45 degrees, under the sky 15 seconds. 



CH. VIII] PREPARATION OF LANTERN SLIDES 215 

3. Cardboard at 45 degrees, lighted by a 40 watt mazda lamp 
above the cardboard 30 seconds. 

4. Cardboard at 45 degrees with a 16 candle-power frosted bulb 
above the cardboard 120 seconds. 

For contact printing with the same negative, 30 cm. (12 in.) from 
the light, if artificial, the following times sufficed: Diffused day- 
light, 2 sec.; Mazda, 40 watt lamp, i sec.; Frosted bulb, 16 c.p. 
lamp, 10 sec.; Petroleum lamp, 10 sec.; Gas mantle, 5 sec. 

332. Rapid preparation of lantern slides. It occasionally 
happens that one needs a lantern slide at very short notice. In 
such a case, the negative can be taken and fixed in the hypo, 
rinsed in water, and put into the camera and a lantern slide 
exposed ( 329). Then the negative can be washed as usual. 
The lantern slide is then developed and fixed, and washed a few 
minutes in water. It is then placed a few moments in 95% alcohol 
or denatured alcohol for dehydration. After removal from the 
alcohol it is dried in a draught or in the current of an electric fan. 
Negatives can be quickly dried in the same way. One can then 
make contact prints. 

333. Type written lantern slides. It frequently happens that 
one desires to project some statement or some table. This can be 
written as stated above ( 316, 321), or the statement or table 
can be made neatly with a typewriter, using a black ribbon. 
Then this can be used just as any other printed matter and a 
photographic lantern slide made from it. 

If in a great hurry one can use the negative form of lantern slide 
and dry quickly ( 332). This will give white letters on a black 
ground ( 329a). (For film slides see 333a). 

333a. Film lantern slides. There has been recently introduced by the 
Eastman Kodak Co., a method of producing lantern slides on celluloid 
films, comparable to film negatives. The celluloid film is quite thick. There 
must be a negative as for glass lantern slides. The film is used in place of a 
lantern-slide plate. The printing is like printing cyco, velox or other paper. 
When the lantern-slide film is dry, after being developed and washed like a film 
negative, it is varnished and placed between two pieces of paper with the 
proper opening for the picture. 

Naturally, these film slides are very light and are not fragile. Unfortunately 
the substance of which the film is composed is inflammable, and therefore the 



216 PREPARATION OF LANTERN SLIDES [Cn. VIII 

334. Mounting lantern slides. In the original method, 
which is still followed to a certain extent, each slide was mounted in 
a w r ooden frame that is, each slide had its own carrier which was 
put in place when it was to be shown (fig. 15). 

For teaching and for many other purposes glass lantern slides 
arc not now put in separate wooden frames, but are covered with a 
clear glass (cover-glass) of the same size and the two bound 
together by adhesive paper. They are far less bulky in this way of 
mounting, although they are not as well protected as in the earlier 
form. 

In mounting them the slides are thoroughly dried, then some 
form of opaque mat or mask is put over the picture on the picture 
side of the transparency or negative. There are on the market 
masks or mats of various shapes and sizes of opening. These may 
be used or masks may be made by using strips of black paper. 

When the mat is in place a cover-glass of exactly the same size 
as the lantern slide is thoroughly cleaned and placed over the 
picture surface of the slide. Then a narrow strip of adhesive paper 
is put all around the edge. This holds the slide and the cover in 
position, and prevents the sharp edges of the glass from cutting the 
fingers when handling the slides. The mat not only cuts out any 
part which is not to be shown, but it separates the cover-glass 
slightly from the picture and prevents rubbing or other injury to it. 
The size and shape of the opening in the mat to give the best effect 
depends upon the picture or other matter on the lantern slide. The 
mat is a kind of frame and like any other frame it should be suited 
in form and size to the object to be shown. 

335. Marking or "spotting" the mounted slides. As 

pointed out in Chapter I ( 23) each slide should have some kind of 

Kodak Company recommend that the film slides be used only with a magic 
lantern having a water-cell (fig. 2, 3). 

Furthermore, even if non-inflammable film were used, it would not do to 
leave those slides in a lantern without a water-cell too long for the heat would 
make the celluloid buckle and get out of shape or char it, although of course 
it would not be set on fire. 

The lightness rind small space 1 required for such slides arc of great advantage, 
but their limitations are so great that for the general, and rough usage of ordi- 
narv lantern slides they are not so well adapted as glass slides. 



CH. VIII] PREPARATION OF LANTERN SLIDES 217 

mark on it so that the operator can put it into the lantern correctly 
without closely inspecting each slide. 

Unfortunately there is no general system of marking slides. The 
method recommended by the British Photographic Club 
(Bayley, p. 78) is to put two white spots on the upper edge of the 
slide (fig. 113). Two spots are necessary for the square slides, but 
for oblong slides one "spot" or mark is sufficient (fig. 112). 

In America it is common to have the mark or spot on the lower 
left hand corner of the slide ( 112), then when the slides are in a 
pile for inserting in the lantern the spot will be turned upward 
(fig. 8) as it must be to give an erect screen image. In the British 
method of "spotting" the slides would have the spots on the lower 
edge when piled up ready for insertion in the lantern. 

336. Coloring lantern slides. Photographic lantern slides 
have been colored from their first production. To do this in the 
best manner possible requires considerable practise and natural 
artistic ability, but any one can color lantern slides sufficiently well 
to add to clearness in teaching for example, veins blue, arteries 
red, etc. All that is needed is a small artist's brush and some of 
the desired color. 

Transparent colors in sets are on the market (see Appendix), or 
one can employ the aqueous stains used in histology. It takes 
some experience to get the right dilution of the color and to put it on 
neatly with the brush. The slide should be held over some white 
paper in a light place so that it is possible to see exactly what is being 
done. The frame for holding slides is a convenience (fig. 113). 

If one wishes to become expert it will be a great help to study the 
works of reference given at the head of this chapter, for they give 
many valuable hints. 

One very important thing for the beginner to do is to test every 
slide that is colored in the lantern to make sure that the colors look 
right in the screen image. Sometimes a slide that looks well to the 
naked eye in daylight will not look well when projected on the 
screen. It is, of course, the screen image that must be satisfactory. 

The early lantern slides were mostly colored with transparent 
oil colors, and then when entirely dry, the slide was mounted in 
Canada balsam, and a cover-glass put on exactly as microscopic 



218 PREPARATION OF LANTERN SLIDES [Cn. VIII 

specimens are now mounted. This gave a very transparent and 
vivid picture. 

337. Labeling lantern slides. Besides the mark or spot as 
guide to inserting the slides in the carrier, every lantern slide should 
have a label stating what it is, and if copied from some book or 
periodical it should give the name of the publication from which 
derived and the number of the figure. 

Slides are also numbered for convenience in arrangement at the 
time of an exhibition. Some workers simply number the slides 
and have no label. This is, of course, feasible for a small collection 
to be used by one individual, but the slides are practically useless 
for any one else unless they are labeled. 

Sometimes slides are numbered, and a catalogue kept with cor- 
responding numbers and a description of the slide. For one 
unfamiliar with the collection the numbers and the cards are not 
easy to put together. Then one is liable to have more than one 
series, and the series are liable to get mixed. With a label on each 
slide, the collection can be made use of by any one. 

338. Storing lantern slides. The problem of storing a large 
collection of lantern slides is a serious one. A still more serious 
problem is to find the slides needed for a given lecture or demon- 
stration. 

A common method of storing is to have a cabinet like that used 
for the card catalogue of libraries, and to put the slides in the draw- 
ers as the catalogue cards are filed. 

One can use name cards to designate groups of slides as they are 
used to group catalogue cards. 

In order to store and make them most easily available for use, Pro- 
fessor George S. Molcr of the department of Physics in Cornell 
University has devised a cabinet which holds the slides in a single 
vertical layer, so that when any holder is pulled out the slides are 
all exhibited, and one can see exactly what the slides are and select 
those desired. 

This seems to the writers of this book, by all odds, the most prac- 
tical cabinet yet devised for safely storing slides and making them 
available with the least trouble and the least waste of time (fig. 120). 



CH. VIII] PREPARATION OF LANTERN SLIDES 219 

339. Troubles in making lantern slides. These are the 
troubles liable to be met in photography. They must be over- 
come by following intelligently the directions for photographic 
work in general and for lantern-slide making in particular. Study 
the directions coming with the lantern-slide plates used. 

In making written slides or diagrams on varnished slides the pen 
will not work well, and the ink will crawl if the varnish is not dry. 




FIG. 120. THE MOLER SECTIONAL LANTERN-SLIDE CABINET. 

(Cut loaned by G. S. Moler). 

This cabinet holds 1200 lantern slides. It consists of a box with twenty 
vertical, sliding frames, each frame holding 60 slides. 

In the picture the cabinet is shown on a table. One of the frames is entirely 
removed and leans against the table leg. One frame is pulled out for examin- 
ing the slides stored in it. 

In coloring lantern slides one must learn to use colors which give 
the correct effect with the artificial light used in projection. A tint 
which does not seem right by daylight may give exactly the desired 
effect by lamp-light. This is why the advice is given to test the 
work frequently in the lantern. 

Remember that there is more danger of getting the lantern 
slides too opaque than not opaque enough. 

Sometimes when being exhibited a lantern slide shows a mist or 
fog spreading over it. This may partly or wholly disappear. 
This is a real fog, and comes from the moisture in the slide, or its 
mounting. If the slides are thoroughly dried before they arc put 
into the lantern this fog does not appear. 



220 



PREPARATION OF LANTERN SLIDES 



[Cn. VIII 



340. Summary of Chapter VIII: 



Do 

1. Use the standard size of 
lantern slides in the country 
where you live ( 312). 

2. Make the lantern slides 
with moderate intensity, then 
they can be used with all lan- 
terns, no matter what the source 
of light (327). 

3. Make the picture small 
enough so that all desired parts 
can be projected (334). 

4. Take pains in mounting 
the slides so that the frame will 
appear suited to the subject 
( 334). 

5. In making slides direct on 
the varnished glass, write finely, 
neatly and clearly ( 316). 

6. Printed or written matter 
on the slide should be large 
enough to be read by all in the 
room (325). 

7. Mark or spot the lantern 
slides so that they can be in- 
serted in the holder without 
hesitation ( 335). 

8. Label every lantern slide 
so that any one can tell what it 
is ( 337)- 

9. Store the lantern slides so 
that they can be found quickly 
(338). 



Do NOT 

1. Do not use odd sized 
pieces of glass to make lantern 
slides on. 

2. Do not make the lantern 
slides so opaque that only the 
best electric lanterns can ex- 
hibit them. 

3. Do not make the picture 
on the slide too large to be 
exhibited. 

4. Do not mount the slides in 
a slovenly, inartistic manner. 



5. Do not use nourishes in 
writing on the varnished slides. 

6. Do not reduce the written 
or printed matter so that it 
cannot be read in the screen 
image. 

7. Do not leave the slides 
unmarked and expect every 
chance operator to insert them 
properly at railroad speed. 

8. Do not leave the lantern 
slides unlabelcd, for no one else 
can make the best use of them. 

9. Do not store the slides in a 
miscellaneous heap. 



CHAPTER IX 
THE PROJECTION MICROSCOPE AND ITS USE 

350. Apparatus and Material for Chapter IX : 

Suitable room with screen, for projection; Projection Micro- 
scope; Sunlight or the electric arc light; Specimens suitable for 
projection ( 399) ; Tools etc., as for Ch. I. 

REFERENCES AND HISTORY 

351. For the history of the origin and development of the 
projection microscope, refer to the appendix at the end of the book. 
In this history will be given many references to the original sources 
of information upon the subject. 

For works dealing with modern micro-projection, the reader is 
advised to consult the works given in 2 of Ch. I. He is especially 
advised to consult the catalogues of Zeiss and the other modern 
makers of projection apparatus, for in them he will find directions 
and suggestions for making the best use of the most modern instru- 
ments. His attention is also especially called to the Journal of 
the Royal Microscopical Society and to the Zeitschrift fiir wis- 
sentschaftliche Mikroskopie. See also the Zeitschrift fur Instru- 
mentenkunde, the English Mechanic and the Scientific American 
with its Supplement. In every volume of these periodicals there 
are almost always articles bearing directly on the problems in- 
volved in Projection. 

GENERAL CONSIDERATION OF THE PROJECTION MICROSCOPE 
352. Similarity of all projection apparatus. All devices for 
projection are fundamentally alike in giving images of brilliantly 
lighted objects. These images are projected upon some reflecting 
surface or screen in a dark room. The projection microscope simply 
gives images of greater enlargement than the other forms of 
apparatus. It imperceptibly merges into the magic lantern, as 
the magic lantern merges into the camera obscura. (Compare 
fig. 121-122). 

221 



222 



THE PROJECTION 7 MICROSCOPE 



[CH. IX 




FIG. 121. PROJECTION MICROSCOPE. 

/, 2 Feeding screws of the arc lamp, 
j Set screw for the upper carbon. 

4 Set screw for holding the stem of the arc 
lamp in the socket on block i. 

5 Set screw for the lower carbon. 

Hc-\r The horizontal, upper carbon. It 
must be made positive (+). 

L The source of light, i. e., the crater of 
the upper carbon. 

Vc The vertical or lower carbon. It is 
negative ( ). 

Axis, Axis, Axis The principal optic axis 
from the positive crater of the arc lamp 
extending through the condenser, the stage 
water-cell, and the microscope to the screen. 

i Condenser 2 The triple condenser for 
receiving and concentrating the light from 
the crater of the arc lamp. 

1 The first element of the condenser which 
renders the diverging light parallel. It con- 
sists of a meniscus next the light and a 
plano-convex lens (compare fig. 105, in). 

2 The second element of the condenser 
which concentrates the parallel beam. 

W Water-cell between the two plano-con- 
vex lenses in the parallel beam of light. 

As a projection microscope uses ob- 
jectives of shorter focus and smaller 
diameter than the magic lantern, 
greater care must be exercised in get- 
ting all the elements, radiant, con- 
denser and projection objective, cen- 
tered along one continuous line or 
axis, and in having the different ele- 
ments the right distance apart. 



CH. IX] 



THE PROJECTION MICROSCOPE 



223 



Micro-projection is simply a refinement of ordinary magic 
lantern projection. If one understands the principles, and has 
mechanical skill to apply them, there is no great difficulty in micro- 
projection. But if ordinary magic lantern projection is unsatisfac- 
tory 7 in untrained hands, micro-projection in such hands is in- 
tolerable. 

This is, however, such a powerful aid to the teacher and the 
lecturer that the time necessary to learn to use it properly is not to 
be counted. With micro-projection the beauties of structure and 



Condenser 



H C 




FIG. 122. MAGIC LANTERN FOR COMPARISON WITH THE PROJECTION 
MICROSCOPE (See fig. 2). 

form are made visible to an entire audience with all their color, 
delicacy and exquisite perfection. 

Furthermore, the teacher or lecturer can indicate on the screen 
the special points to be noted, and feel confident that his auditors 
see the special features and do not get confused by the mass of 
details, as when looking into a microscope. Often too, the most 
interesting and important structures in a specimen are not so 
striking as some less important detail, and the important points are 
likely to be missed unless pointed out. 

353. Limitation of the Projection Microscope. Perfect and 
useful as the projection microscope is, it is limited in its powers. 
One can show with full satisfaction to a large audience (200 to 
1000) only those details which an experienced observer can see by 



224 MICRO-PROJECTION FOR LARGE CLASSES [Ca. IX 

looking directly into a compound microscope supplied with a low 
ocular and a 16 mm. objective. For a small audience near the 
screen higher powers are satisfactory (see 401). 

354. Size of specimens for projection. To meet the require- 
ments of teaching and demonstration the modern scientific man 
and public lecturer should be able to commence with the projection 
microscope where the magic lantern leaves off, and carry the pro- 
jection to the smallest size adapted to micro-projection; that is, 
from a specimen 60 mm. in diameter to one of half a millimeter or 
less in size. This requires an opening in the stage slightly larger 
than the largest specimen, that is, at least 65 mm. in diameter. 

CHARACTER AND RANGE OF PROJECTION OBJECTIVES FOR DEMON- 
STRATION TO LARGE CLASSES 

355. Objectives from 125 mm. to 4 mm. equivalent focus are 
especially useful in micro-projection. The powers of 125, 100, 75, 
50, and 25 mm. equivalent focus, and in some cases those of 20 and 
1 6 mm., are constructed on the plan of photographic objectives 
(fig. 123). These are always to be used without an ocular, and 
their iris diaphragms are wide open. 

At the present time the low objectives used in ordinary micro- 
scopic observation are also used in projection. The field is not 
flat, as with the micro-planar and other forms of photo-micro- 
graphic objectives, but they are much cheaper and the screen 
images are very brilliant. Formerly many of the objectives used 
in projection were made especially for that purpose. They gave 
very brilliant, flat fields over a narrow angle, but they were neither 
satisfactory for ordinary microscopic observation nor for 
photography. 

Most of the projection with the microscope is, however, accom- 
plished with objectives of about the following range: 50 mm., 
1 6 mm., and 8 mm. With these in a triple nose-piece or revolver, 
the projection microscope can accomplish great things, especially 
if assisted occasionally by amplifiers. For an audience of 2 50 to 500 
and a screen distance from 7.5 to 10 meters (25 to 33 ft.) the mag- 
nifications will range from about i 50 to 3000 diameters ( 391). 



CH. IX] MICRO-PROJECTION FOR LARGE CLASSES 



225 



For a larger audience and a correspondingly larger room the 
screen distance might be made 15 to 20 meters (50 to 65 ft.), and 
the magnification raised from 250 at the lower limit up to about 
5,000 diameters at the upper limit. The smaller room enables 
one to get more brilliant screen images, and to use a wider range of 
objects (see table of magnifications 391). In the smaller room 
the screen should be at least 4 meters (12-13 feet) square, and in 
the larger room 5-6 meters (15-20 feet) square. 




B 



FIG. 123. DIAGRAMS SHOWING THE CONSTRUCTION OF OBJECTIVES FOR 
MICRO-PROJECTION AND FOR PHOTOGRAPHY. 

(From the Catalogues of Zciss, Leitz, and the Bausch & Lomb Optical Co.). 

A Microsummar of Leitz. 

B Microplanar of Zciss. 

C Microtessar of the Bausch & Lomb Optical Co. 

When used for micro-projection the diaphragm is wide open and no ocular is 
employed. 

In the diagram of the Microtessar, F represents the front lens, d the dia- 
phragm, and B the back combination of the objective. The arrow indicates 
the direction of the light. 

In articles and books upon projection, it is advocated sometimes, 
that oil or water immersion objectives as high as i .5 or 2 mm. should 
be used for class demonstration. 

There is no doubt that brilliant images with short screen dis- 
tances can be obtained with high power objectives, but such pro- 
jection is only applicable for small numbers ; and if fine details are 
to be seen, the observer must be very close to the screen. Further- 
more, no screen image in its finest details is equal to that which 
one gets in looking directly into a compound microscope. (For 
high power projection sec 401). 

If it is high magnification that is desired, it is vastly better to use 
lower objectives with an amplifier ( 356, fig. 126). The lower 



226 



MICRO-PROJECTION FOR LARGE CLASSES [Cn. IX 



objective with larger lenses admits much more light, hence the 
screen image will be brighter. For example, suppose it were 
desired to obtain the magnification which is given by a 2 mm. objec- 
tive, it would be much better to use a 4 mm. objective and an 
amplifier doubling the size of the real image. This would make the 
screen image of the same magnification as the 2 mm. would give, 
and it would be far brighter and show a larger field. In like 
manner and for the same reason, it is better to use an 8 mm. objec- 
tive and an amplifier, than a 4 mm. objective without the amplifier 
(but see 401). 






A B C D 

FIG. 124. FIGURES SHOWING THE GENERAL CONSTRUCTION OF MICROSCOPE 

OBJECTIVES. 

A Low power objective of a single combination (50-30 mm. equivalent 
focus). 

B, C Medium power objectives with two combinations (25-12 mm. 
equivalent focus). Sometimes the front combination is composed of two and 
sometimes of three lenses as shown. 

D High power objective (8 to 2 mm. equivalent focus). Usually the front 
combination is of a single lens, the others of two or three lenses as shown. 
Many high power objectives have but three combinations. 
(D is from Voigtlander's Catalogue). 

The writers have found that in projection for actual class demon- 
strations, objectives of higher power than 4 mm. arc unsatisfactory. 
We believe also that the purpose of class-room projection is not 
the demonstration and study of minute details which require that 
the observer should be close to the screen image, but the general 
outlines and broad features which can be seen clearly at a distance 
when suitably magnified. 



CH. IX] MICRO-PROJECTION FOR LARGE CLASSES 227 

The fresh blood corpuscles of man, for example, are about 7.5/4 
in diameter. To see these as discs on a screen at a distance of 10 
meters would require a magnification of 4,000 and preferably of 
8,000 diameters. With such a high magnification the sharpness of 
the outline, and the distinction between the corpuscles and the 
medium in which they float is almost lost, and there is nothing but 
a vague haze with shadowy outlines. If one goes up closer to the 
screen to see the images well, one will be sorely disappointed, for 
they are vague in outline and wholly unsatisfactory as compared 
with the appearance gained by looking directly into a microscope 
( 355a). 

355a. Visibility of objects or their magnified images. It has been found 
by careful observation and experiment that the most sensitive part of the eye 
is in the fovea centralis or yellow spot; and that in order to see two points, by 
the fovea, as separate, they must be far enough apart so that the visual angle 
is one minute. If the visual angle is less than one minute, two points appear to 
most eyes as one. 

The question now is, how far separated must the parts of an object be in 
millimeters or inches in order that the form of the object can be distinguished. 
To answer this it is necessary to know the actual length of the one minute of arc 
when the eye is at different distances. 

To determine the length of one minute of arc in any case, the eye is con- 
sidered to be at the center of a circle and the object at the circumference, and 
no matter how great the visual distance, the object must subtend one minute 
of the arc of the circle of which the visual distance is the radius in order to have 
its parts distinguishable. 

To determine the actual length in millimeters or inches of one minute of arc 
in any circle, it is only necessary to remember that the circumference of a circle 
is 6.2832 times its radius and that it is divided into 360 degrees or 21,600 
minutes (fig. 125). 

If, now, the radius of the circle, or the distance of the eye from the object is 
I meter, the circumference of the circle will be 6.2832 meters or 6,283.2 milli- 
meters. As there are 21,600 minutes in the circumference, the length of one 
minute is 6,283.2 mm. --.- 21,600 = .2908 mm. or approximately .3 mm. That 
is, with the eye at one meter distance, the parts of an object should be separated 
.3 mm. to be seen as distinct points. 

For the standard distance of distinct vision (25 cm.), used in microscopic 
magnification, the object must be Kth this size or .075 mm.; and for a dis- 
tance of i o meters it must be 10 times as great or 3 millimeters, and for 6 meters, 
the distance used for testing vision, it must be .3x6 = 1.8 mm. 

A greater separation of the points is desirable for the most accurate deter- 
mination, but those given above are the minimum for most observers. 

Now to apply the above to the magnification necessary for a screen image of 
the human blood corpuscle which has a size of 7.5," (.0075 millimeters; .000295 
inch). To give the necessary sized screen image of .3 mm.; .075 mm. and 3 
mm. at distances of i meter, ^th meter, and 10 meters, it is only necessary to 
divide the size of the screen image in each case by the size of the object (7.5^ 
or .0075 mm.). 



228 MICRO-PROJECTION FOR LARGE CLASSES [Cn. IX 




FIG. 125. DIAGRAM SHOWING VISUAL ANGLE. 

The nodal point or optic center of the eye is placed at the center of the circle, 
and the rays from the extremities of the object which cross at this nodal point 
show the visual angle. 

It is clearly seen from the diagram that the object must increase in length 
in direct proportion to its distance from the eye if the visual angle remains 
constant. 

Visual Angle The angle between the lines extending from the extremities 
of the visible object and crossing at the nodal point (n) of the eye. 

Axis The straight line extending along the principal optic axis of the eye 
to the visible object on one side and to the retina on the other side of the nodal 
point (n). 

n Nodal point or optic center of the eye. 

ri Retinal image. The size of the retinal image of a given object depends 
upon the visual angle and the visual angle depends upon the distance of the 
object from the nodal point. 



For I meter (.3 mm. -=- .0075 = 400 diameters magnification). 
For YJ, meter (.075 -: .0075 = 100 diameters magnification). 
For 10 meters (3 -: .0075 = 4,000 diameters magnification). 
For anything like a satisfactory view of the corpuscles, it would be desirable 
to double these magnifications. 



CH. IX] 



MICRO-PROJECTION WITH AMPLIFIERS 



229 



356. Amplifiers. An amplifier is a concave lens or combina- 
tion producing divergence instead of convergence of light rays, 
hence placing an amplifier in the path of the image-forming rays 
from the objective produces a larger image (fig. 126), and there is 
little loss in light. It should be made as great in diameter as the 
large tube (fig. 121) of the microscope will receive to avoid cut- 
ting down the field, and should be mounted in a short tube which 
can be easily slipped into a cloth-lined collar screwed into the 
end of the microscope tube (fig. 133). 

The amplifiers most generally useful are of -5 and -10 diopters. 
The average increase in magnification given by the -5 diopter 
amplifier is 1.7 and that given by the -10 diopter is 2.5 (see 356a). 



Object 



Objective 




Microscooe Tube 122 x 46 mm. ****, ~~~~^-- 

FIG. 126. AMPLIFIER FOR PROJECTION. 

Object The object to be projected. 

Objective The projection objective. 

Axis Optic axis of the apparatus. 

A mplifier The concave lens diverging the rays from the objective and thus 
increasing the screen image. 

Images The ones with broken lines show the images with a - s diopter and a 
-10 diopter lens. The full lines show the image which the objective alone 
would give. 

The microscope tube is 122 mm. (4.8 in.) long and 48 mm. (1.9 in) in 
diameter. 



356a, 403a. Diopter, Dioptre, Dioptry. For spectacle lenses especiallv 
this is the unit of strength. It is the strength of a lens of i meter principal 
focus. 

As the focal length of a lens varies inversely as its power, the focal length of a 
lens of 2 diopters is one-half as great as the standard, hence it has a focal length 
of y 2 meter; and one of 10 diopters has a focal length of i/io meter and so on 

tor lenses having a strength less than the standard of i meter the focal 
length will also be inversely as the power, and hence a Y* diopter lens will have 
a focus of 2 meters and a i/ioth diopter lens has a focus of 10 meters. In 
general, the less the dioptry or strength the longer is the focus, and the greater 
the dioptry or strength the shorter is the principal focus. 

Convex lenses with a real principal focus arc indicated by the plus sign ( + ). 



230 



MICRO-PROJECTION WITH OCULARS [Cn. IX 




FIG. 127. HUYGENIAN OCULAR IN SECTION. 
(From The Microscope). 

F. L. Field lens. This aids the objective in forming a real image. 

D Diaphragm in the ocular. It is at this level that the real image is 
formed in ordinary microscopic observation. 

E. L. The eye lens. In projection this acts like an objective and projects 
upon the screen an image of the real image (see fig. 207). 

A xis The optic axis of the microscope. 

E. P. Eye-point or Ramsden's circle. 



357. Projection oculars. Any ocular may be used for pro- 
jection. The lower powers, x 2, X3, x 4, x 6, ( 357a) are 
better than the higher powers, for they cut down the field less, there 
is less loss of light, and there is not an inordinate magnification. 

Concave lenses having a virtual focus are indicated by the minus sign ( ) . 

If the dioptry of a lens is given, to find the principal focus: divide i meter 
by the dioptry. For example, the dioptry of the amplifiers mentioned above 
( 356) is 5 for one and 10 for the other. Their foci are then i meter, 

~5 
i meter. That is, they are concave lenses of 1/5 and i/io of a meter focus. 

-10 

On the other hand, to find the dioptry of a lens whose principal focus is known, 
divide i meter by the principal focus and the result will represent the dioptry 
of the given lens. Taking the same case as before where the amplifiers have 



principal foci of 1/5 and i/io meter, 



As the lenses are 



known to be concave, the minus sign is placed before the dioptry: 5, 10 
diopters. 

The increase in magnification given by the amplifiers, 5, 10 was found 
to average 1.7 for the 5 and 2.5 for the 10. The average was obtained by 
considering all the screen distances and all the different objectives shown in the 
table, 391. See also 392a. 



CH. IX] MICRO- PROJECTION WITH OCULARS 231 

In using the ordinary oculars a small tube must be screwed into 
the large microscope tube as for ordinary observation (fig. 147, 

197). 

Special oculars have been designed for projection. Some, like 
those of Zeiss (fig. 128) give sharp brilliant images, but the field 
is very small. Williams, Brown and Earle have a very large pro- 
No. 2. 



No. 4. 




FIG. 128. PROJECTION OCULARS OF ZEISS. 
(From Zeiss' Catalogue, No. jo). 

A section has been removed to show the construction. Both are of the 
negative form. 

The eye lens is in a smaller tube with spiral movement to enable the operator 
to focus the image of the diaphragm of the ocular sharply on the screen. 
Below are shown in face view the upper ends of the oculars with their graduated 
circles. By noting the position in any experiment it is easy to set the position 
exactly the same if the experiment is to be repeated. 

No. 2, No. 4 These numbers indicate that the ocular magnifies the image 
two or four times (see 391). 

jection ocular of the Huygenian form which magnifies about twice. 
On account of the loss of light and the restriction of the field of 
view, the writers of this book do not advocate the use of oculars for 
ordinary micro-projection, but sec 401. 

357a. Designation of oculars. At the present time an ocular is usually 
designated by the increase in magnification it gives a microscopic image when 
the microscope is used in the ordinary way. For example, if the objective alone 
would give an image 10 times as long as the object, then an ocular x 2 should 
double that size, thus giving an image magnified 20 times, and an ocular x 4, 
an image magnified 40 times and so on. 



232 



MICRO-PROJECTION WITH OCULARS 



ICH. IX 



358. Micrometer ocular for demonstration. It is so difficult 
for most students to understand the workings of the ocular micro- 
meter, that it is of great help to them to use a micrometer ocular 
like fig. 130 to 131 on the projection microscope, then the object 
and micrometer lines can be projected together by suitably adjust- 
ing the eye-lens of the ocular. A stage micrometer might also be 
used as object and the students shown, all together, how to deter- 
mine the ocular micrometer valuation (see Gage, The Microscope). 



Oculir lo 2 




FIG. 129. COMPENSATION OCULARS. 

(From Zeiss' Catalogue, No. jo). 

A section has been removed to show the construction. 
The numbers 2, 4, 8, /2, 18, indicate the magnification of each 
ocular (see 357a, 391 a). 

359. Substage condensers. The writers believe, from their 
experience and experiments in photometry under the different 
conditions, that it is better to use for illumination only the large 
condenser (fig. 121). 

The use of a substage condenser is for either one of two purposes : 
(i) to enable the position of the object and the projection objective 

The average increase in magnification given by the different oculars with the 
different objectives and screen distances shown in the table ( 377) is as follows: 
Projection ocular X2 gives a magnification of ................... 1.99 

*4 " ................... 3-^9 

Compensation " X2 " ................... 2.05 



Huygenian x4 " ................... 4.2 1 

From these figures it is seen that the increase in magnification for projection 
work can be closely enough approximated by multiplying the image given by 
the objective alone by the number designating the ocular, i. e., 2 or 4. 

If very precise results are desired, one must use a stage micrometer and pro- 
ceed as described in 391 a. 



CH. IX] PROJECTION WITH SUBSTAGE CONDENSER 



233 



to be different from what it would be with the main condenser only ; 
or (2) to make the aperture of the illuminating cone correspond 
with that of the objective. 

The positional reason (i) can only have weight when combined 
apparatus is used, that is, when a magic lantern objective as well 
as microscopic objectives are used without changing the distance 
between the main condenser of the microscope or the magic lantern 
objective. 




FIG. 



130. OCULAR MICROMETER WITH MOVABLE SCALE. 
(Cut loaned by the Spencer Lens Co.). 



This is a Huygcnian ocular with a 5 mm. scale divided into twenty K" mm. 
intervals. The pitch of the screw moving the scale is Y\ mm., therefore one 
complete revolution of the drum moves the scale one interval or '--4 mm. The 
drum is divided into 100 graduations thus enabling one to measure looth of 
an interval on the micrometer scale. This ocular micrometer combines the 
advantages of the ocular micrometer with fixed scale and the filar micrometer. 
To complete the measurement of an object not exactly between any two 
micrometer lines the drum need be revolved only partly around. 

With reference to the aperture (2) it is one of the fundamental 
laws of microscopic vision that the brilliancy and clearness of 
details depend largely upon the aperture of the light which illumin- 
ates the object, and which passes through the objective to form the 
retinal or the screen image. As the numerical aperture of objec- 
tives varies greatly it is necessary, if the clearest and most brilliant 
images are to be produced, to light the object with a numerical 
aperture equal to that of the objective. Where substage con- 
densers are used arrangements must be made for this. 



234 



PROJECTION WITH SUBSTAGE CONDENSER [Cn. IX 




FIG. 131. FILAR MICROMETER OCULAR. 
(Cut loaned by the Bausch & Lomb Optical Co.}. 

This filar micrometer ocular is of the Ramsden type and consists of a positive- 
ocular with a movable hair line and two reference lines at right angles to each 
other as shown in A. The movable line must be carried over the entire length 
of the object to be measured by rotating the drum. 

A Field of the filar micrometer showing the movable and the cross lines, 
and the comb. The teeth serve to measure the total revolutions of the'drum. 




FIG. 132. ILLUMINATING OBJECTS OF VARIOUS SIZES ix MICRO-PROJEC 

TION WITH THE MAIN CoNDEXSER OxLY. 

The object must be put in the cone of light at a point where it will be fully 
illuminated. 

For high powers it will be at or very near the focus (/). For larger objects 
and low powers the object is at 2 or j, or even closer to the condenser face. 

Arc Supply The right-angled carbons of the arc lamp. 

L 1 L 2 The first and second elements of the triple condenser. 

Water- Cell The water-cell for absorbing radiant heat. It is in the parallel 
beam between the first and second elements of the condenser. 

Axis The principal optic axis on which all the parts are centered. 

If only the main condenser is used (fig. 121), the cone of light 
from the condenser must be sufficient to fill the aperture of the 
projection objective. This requires that the second clement of the 



CH. IX] PROJECTION WITH SUBSTAGE CONDENSER 235 

main condenser (fig. 132 La) have a focus of 150 to 200 mm. (6 to 8 
inches). With such a main condenser one can do successful pro- 
jection with objectives from 125 to 4 mm. focus. The aperture 
will not be completely filled in the 8, 6 and 4 mm. objectives, but 
brilliant screen images are obtained even with them for a 7 . 5 meter 
(25 ft.) screen and 12 amperes of direct current. One can also use 
a -5 diopter amplifier when good specimens are projected. (For 
the position of the objective and specimen see 376). 

With a substage condenser there is a great loss of light from 
reflection and absorption so that the increased aperture hardly 
compensates for it, and the increased detail is lost for the observers 
are too far from the screen to see them (see 35ga). 

For special demonstrations and for drawing where the observers 
are very close to the screen, the substage condenser and also an 
ocular are advantageous, and for fine details, necessary (see 401, 
477). 

SUITABLE ROOM AND SCREEN FOR MICRO-PROJECTION 

360. From the small size of the objective for micro-projection 
the image on the screen cannot be made as bright as with the magic 
lantern, hence it is necessary in micro-projection to have a room 
that can be made very dark ; and the devices for cutting out stray 
light, bellows, objective hood and shield must be efficient (fig. 
133. 139)- 



359a. i. Wright, p. 212, says: "The iris of the substage condenser is 
opened or closed until the best effect is produced." This can mean only that 
not the whole cone of light is used in some cases. 

2. To determine the amount of aperture of the objective used in projection, 
take a thick piece of smoked mica or combine brown and blue, or deep red and 
blue, or red and green glass and put them over the front of the objective to 
soften the light. Or one might hold one of these light softeners just in front 
of the eye. Then in any given case look along the microscope tube directly 
toward the light, and the aperture of the objective actually filled by the enter- 
ing cone of light can be seen. If the entire aperture is used, the back lens of 
the objective will be filled with light; if only a part of the aperture, then there 
will be a central brilliant circle and a dark zone of glass surrounding it (fig. 151). 

It must be remembered too that the large specimen cooler (fig. 121, 134) 
cannot be used with a substage condenser; and in our opinion this overbalances 
any advantage that the substage condenser might yield for demonstrations to 
large classes. 



236 MICRO-PROJECTION WITH DIRECT CURRENT [Cn. IX 

The screen must be as reflecting as possible. Nothing has ever 
yet exceeded in satisfactory quality a smooth, dull, white, wall. 
For a full discussion of screens see Ch. XII, 621. 

MICRO-PROJECTION WITH THE DIRECT CURRENT ARC LAMP AS THE 

LIGHT SOURCE 

361. Arc lamp and wiring for the same. The direct current 
arc light is the only fully satisfactory artificial light known at 
present for micro-projection. Hence it will be taken as the 
standard, as with the magic lantern (Ch. I). Furthermore, as the 
upper carbon is always made positive and hence is the source of 
light, this carbon is made horizontal and the crater faces the con- 
denser and is in the optic axis. That is, for micro-projection 
we take the right-angled arc lamp as the standard (fig. 3, 121). 

The wiring, rheostat and ammeter are as with the direct current 
magic lantern radiant, (figs. 2, 3, 133). The rheostat should be an 
adjustable one. The ammeter can be omitted, but it is more 
important than with the magic lantern, for the conditions of 
micro-projection must be made as nearly perfect as possible. With 
the ammeter one can tell instantly whether the proper amount of 
current is flowing. If there is sufficient current the light should be 
satisfactory, or if it is not satisfactory it will be due to some fault 
in optical adjustment. The ammeter is urged upon all users of the 
projection microscope because the tendency is to run in more and 
more current if the projection is unsatisfactory, hoping by pure 
brute strength, so to speak, to overcome difficulties due to improper 
adjustment. In case one cannot afford an ammeter, then the next 
best thing is, when installing the apparatus, to measure the current 
flowing through the arc with the different settings of the adjustable 
rheostat, and to mark these values on the rheostat dial. One can 
then set the rheostat at the proper amperage for the given projec- 
tion; but as the voltage on the line is subject to variation, one 
cannot be sure that the proper current is flowing at any given 
moment unless an ammeter is present to indicate the amount. 
With many lighting circuits, the fluctuations in voltage are very 
small, and one can be reasonably sure of getting the current indi- 



CH. IX] MICRO-PROJECTION WITH DIRECT CURRENT 237 

cated on the dial of the rheostat. When current is drawn from an 
overloaded power line, however, the voltage fluctuations are often 
so great that an ammeter, as well as an adjustable rheostat, 
is a necessity. 

362. Fine adjustment for the arc lamp. For micro-projec- 
tion it is absolutely necessary to have fine adjustments on the arc 
lamp so that the position of the crater can be changed slightly 
during an exhibition. In the burning of the carbons there is a 
slight shift in position of the crater even with soft-cored carbons. 
The crater may be in perfect alignment to start with, and by the 
shifting as the carbon burns away it may get far enough outside 
the longitudinal axis on which the apparatus is placed to spoil the 
light on the screen. This is emphatically true for high powers (16 
mm. and higher). If now there are fine adjustments on the lamp 
(fig. 3, 146), by which the crater can be slightly raised or lowered 
or turned toward the right or left, compensation for this shifting 
can be made, and the most brilliant part of the crater kept strictly 
in the axis where it must be to give satisfactory illumination. 
Furthermore, it is necessary to have an independent adjustment 
for one or both of the carbons, so that one or both carbons may be 
moved independently. This is because the carbons are liable to 
wear away somewhat unequally, and some one of the mal-positions 
shown in fig. 24, 25 would occur if the carbons were not adjustable. 

363. Condenser. The triple form with a meniscus next the 
radiant (fig. 121, 132) is very satisfactory for micro-projection, 
although many use the double form (fig. 146) with success. As 
the objectives used for projection with the microscope are of short 
focus and rather large aperture the final element of the condenser 
used to bring the light to a focus should not be of too great focal 
length. A focus of 150-200 mm. (6-8 in.) is a good average for 
the condenser with the objectives usually employed (125 to 4 mm., 
355)- See 401 for condenser with substagc condenser. 

364. Water-Cell to prevent overheating. For micro-projec- 
tion a water-cell in connection with the large condenser is a neces- 
sity. It absorbs most of the radiant energy in the infra-red part 



238 MICRO-PROJECTION WITH DIRECT CURRENT [Cn. IX 




FIG. 133. PROJECTION MICROSCOPE WITH AMPLIFIER. 

This picture shows the projection microscope arranged for use in a lecture 
room. 

Commencing at the left : 

The supply wires to the table switch. 

A The ammeter to indicate the amount of current. It is along one wire 
(in series). 

R The adjustable rheostat. It is along one wire. 

10-20 These figures indicate that the rheostat is adjustable; the lowest 
current allowed to flow being 10 amperes and the highest 20 amperes. The 
arrow indicates the direction to turn the knob to increase the current. 

The arc lamp in the lamp-house. This is the three- wire, automatic arc lamp 
of the Bausch & Lomb Optical Co. 

The wiring is shown to be: 

One wire from the negative pole of the switch to the pole for the lower carbon. 

One wire passes from the positive pole of the switch to the middle binding 
post of the motor mechanism of the automatic lamp. The current for the 
motor does not traverse the rheostat. 

One wire passes from the positive pole of the switch to the ammeter, to the 
rheostat and from the rheostat to the positive ( + ) binding screw of the art- 
lamp. 



CH. IX] MICRO-PROJECTION WITH DIRECT CURRENT 239 

The metal lamp-house is semi-transparent as it was in position during only 
a part of the exposure for the photograph. 

The condenser and water-cell are connected to the stage by a bellows to 
exclude stray light. 

The microscope shows the objectives on a revolving nose-piece and behind 
them a metal shield to keep stray light from the screen. 

An amplifier is shown in place, at the end of the large tube of the microscope. 

The arc lamp, condenser, stage and microscope are each on an independent 
block which moves along the optical bench on the single baseboard. The 
vertical white lines on the baseboard indicate the position of the various blocks 
for the optical combination here shown. 

On the front legs of the table is the adjustable drawing shelf upon which are 
demonstration preparations. 

The scale of this picture is shown by the 10 centimeter rule just above the 
table drawer at the right. 

of the spectrum and thus helps to avoid the overheating which 
would result if all of this energy remained. The best position of 
the water-cell is between the first and second elements of the con- 
denser, where the rays are practically parallel (fig. 121). For 
further discussion of the avoidance of heating the specimens, see 
852. 

365. Stage for specimens. The stage should be of ample 
size, and should have an opening sufficiently large for the largest 
specimens to be used in micro-projection, that is, not less than 65 
mm. (2 l /2 in.) square. 

366. Mechanical stage. If serial sections are to be used 
with the apparatus then the stage should be supplied with a 
mechanical stage of great range, that is about 50 x 65 mm. This 
is about the maximum range for the sections mounted on slides 
50 x 75 mm. (2x3 in.) (fig. 135, 136). , 

367. Stage cooling device. While the large water-cell in 
connection with the condenser absorbs practically all the long 
waves of radiant energy that can be absorbed by water, it is very 
desirable, and for many specimens necessary, to have some device 
for carrying off the heat developed in the specimen itself by the 
absorbed light. The most practical stage cooling device is a stage 
water-cell. The one found very efficient and satisfactory in every 
way is shown in fig. 121, 134. The specimen rests directly against 
the glass side of the water-cell and is cooled bv conduction. Manv 



240 



MICRO-PROJECTION WITH DIRECT CURRENT [Cn. IX 



B 




D 



C 




FIG. 134. FACE VIEW OF THE 
STAGE OF THE PROJECTION 
MICROSCOPE AND SECTION- 
AL VIEW OF THE STAGE 
WATER-CELL. 

(About half size) . 

(From The Microscope, Ninth 
Edition, 1904). 

A Sectional view of the 
stage with the stage water- 
cell. 

S Metal part of the stage 
in section. 

S w. Stage water-cell. 

gsf Glass front of the stage 
water-cell. The microscopic 
specimen rests directly upon 
the glass front, and heat from 
the specimen is conveyed 
away by conduction. 

B Face view of the metal 
part of the stage of the projec- 
tion microscope (fig. 121), 
and the optic bench. In this 
case the base (E) with V's is 
of cast iron as is also the 
block (D). Both were pre- 
pared on a lathe (Compare 

fig- 158, 159). 

E End view of the guide 
piece with V's. 

D Apparatus block. 

C Post of the stage in the 
block socket. Two set screws 
hold the post in place. It is 
better to use but a single 
screw for this. 

The stage proper has a very 
large opening, and the water- 
cell inserted in this opening 
permits of the demonstration 
of specimens up to 65 mm. in 
diameter. 

specimens like those of 
the nervous system 
stained with Wei^ert's 
hcn~.atoxylin, or by the 
Gol^i ir.cthod absorb a 
<rreat deal of the li<rht 



CH. IX] MICRO-PROJECTION WITH DIRECT CURRENT 



241 



falling upon them, and hence, following the law of the conserva- 
tion of energy, all this absorbed light is transformed into heat. 
The darker the specimen the more light is absorbed, and the quicker 
it will be spoiled by overheating. The stage water-cell against 
which the specimen rests conducts this heat away, in part, and 
makes it possible to exhibit the specimen a longer time (see 852). 




FIG. 135. MECHANICAL STACK OF GREAT RANGE. 

(Cut loaned by the Spencer Lens Co.). 

This can be clamped to any rectangular microscope stage and as no part of 
the clamp extends above the stage the full range of 85 by 65 mm. is available 
and slides 50 x 75 mm. (2 x 3 inches) can be examined to the edges. This is 
of the greatest convenience in examining serial sections, and also in projecting 
them on the screen. 

368. Microscope-tube, and focusing device. If a tube 
for receiving the objective is used it should be a large one, (fig. 
121, 145). The small tubes used on most microscopes, and on all 
when using an ocular, cut down the field too greatly (fig. 137, 147). 
The tube should be short, that is, about 9 to 10 cm. (4 in.) long, and 
4 to 5 cm. (2 in.) in diameter. There should be coarse and fine 
adjustments as for the ordinary microscope (fig. 121). 

369. Mounting of objectives of low power. For the lowest 
powers (125 to 75 mm. equivalent focus) it is better to have no 
tube at all, but to have a black shield about 15 cm. (6 in.) in diam- 



242 



BLACK APPARATUS FOR MICRO-PROJECTION [Cn. IX 



eter into the center of which is screwed the objective (fig. 138), then 
the field is not at all restricted. The low power objectives can be 
focused easily by moving their supports back and forth along the 
optical bench by hand (fig. 158-159). 




FIG. 136. MECHANICAL STAGE OF WIDE RANGE. 

(Cut loaned by the Bausch c? Lomb Optical Co.). 

This mechanical stage can be attached to any microscope with square stage, 
and it permits the use of large slides. The right to left scale is 80 mm. and the 
front to back one 58 mm. The actual range available depends on the size of 
the stage of the microscope. 

BLACKENED APPARATUS 

370. The light necessary for micro-projection is so dazzling 
that it should IDC kept strictly within the projection apparatus by 
means of a proper lamp-house and bellows, so that the only light 
which finally reaches the screen is that which passes through the 
projection objective. But this ideal condition cannot be wholly 
realized in practice, hence the necessity of making the outside of 
the entire apparatus, from lamp-house to microscope tube, dull 
black. Then any escaping light will not be reflected from polished 
surfaces and scatter light into the room, on the one hand, or blind 
the eyes of the operator and annoy the auditors on the other. 
Projection apparatus found in institutions, in many cases, have a 
finish of polished brass or nickel. If the operator cannot focus 



CH. IX] BLACK APPARATUS FOR MICRO-PROJECTION 



243 



properly and has ill success in general, there is no wonder, as he has 
blinding reflections constantly in his eyes. With a dull black finish 
for all outside surfaces, if the apparatus is properly built, this 
defect will be abolished. Polished black finish will not answer, for 
it reflects almost perfectly. The finish must be dull, dead, or luster- 
less, then the light will be mostly absorbed, and so small a part 
reflected that no inconvenience is produced. 

371. Blackening the interior of projection apparatus. As 

with the exterior of projection apparatus, so the interior of all the 
parts should be dull black to avoid internal reflections and conse- 
quent confusion. This is especially true of the objective mount- 
ings, the tube of the microscope and the amplifier tube. Lewis 




FIG. 137. DIAGRAM TO SHOW THE SIZE OF IMAGE WITH THE SAME OBJEC- 
TIVE AND DIFFERENT LENGTH AND DIAMETER OF MICROSCOPE TUBE. 

Objects The different lengths of object shown. 

Objective The projection objective. 

Microscope Tube Microscope tubes with diameters of 48, 30 and 23 milli- 
meters. 

i, 2 Rays which are stopped by the largest tube. 

Ray 3 The marginal ray allowed to pass by the largest (48 mm.) tube. 

Ray 4 The extreme marginal ray allowed to pass by the 30 mm. tube. 

Ray 5 The marginal ray allowed to pass the 23 mm. tube. 

Axis The optic axis. 

Images The one in full lines is for the smallest tube. The others in 
broken lines for the tubes of larger size. 

By tracing back to the specimen it is seen that the larger tubes show corre- 
spondingly more of the object, the projection objective remaining the same. 



244 BLACK APPARATUS FOR MICRO-PROJECTION [Cn. IX 



FIG. 138. PROJECTION WITH PHOTOGRAPHIC 
OBJECTIVES OF 75 TO 125 MM. Focus. 




Commencing at the left: 

The supply in this case is from the house circuit for a current of five amperes. 

There is first a separable attachment plug in the lamp socket. On the table 
is a separable extension. This is to serve as a safe switch for turning the cur- 
rent on and off. 

R Small rheostat for five ampere currents. It is in scries, along one wire. 
In this case it is the positive wire, if direct current is used, and goes to the bind- 
ing post of the upper or horizontal carbon. 

The other wire extends between the binding post of the arc lamp and the 
separable extension. 

The arc lamp with small carbons, in the metal lamp-house. The lamp- 
house appears transparent as it was in place during only a part of the exposure. 

Following the lamp-house is the triple condenser and water-cell (fig. 122). 

The stage with the stage water-cell and the mechanical stage of great range 
(fig. 121, 135). 

Support for the photographic projection objective. 

All the parts are supported by posts and blocks and all move independently 
on the baseboard with track. The vertical white lines on the baseboard 
indicate the proper relative positions of the different blocks. 

At the extreme right is shown the adjustable drawing shelf attached to the 
legs of the table. On this shelf is the projection microscope with three objec- 
tives in the revolving nose-piece. 

The shield behind the objectives is to prevent stray light from reaching the 
screen. Demonstration preparations are also shown in the slide box on the 
shelf. 

The projection table with the drawer for holding apparatus is shown with 
the legs partly removed. The entire table drawn to scale is shown in fig. 182. 
In this picture the scale is shown by the 10 centimeter rule just above the 
drawer at the right. 



CH. IX] BLACK APPARATUS FOR MICRO-PROJECTION 245 

Wright (p. 194) in speaking of the necessity of a dull finish in the 
interior of objectives says : "I may add here that some really good 
lenses [objectives] when used with brilliant light such as projection 
demands, give a "mist" over the image purely from flare, or reflec- 
tion in the lens mount, and which is removed by careful blacken- 

ing." 

Finally, there may be a bright spot or "ghost" in the screen image 
from the internal reflections of a shiny microscope tube, especially 
if the tube is small. If an ocular is used this ghost usually dis- 
appears. It can also be avoided by having the interior of the 
microscope tube a dull black ( 37ia). 



Objective Hood 




FIG. 139. PROJECTION OBJECTIVE WITH 
BLACK METAL HOOD. 

372. Hoods for projection objectives. Usually the ends of 
objectives are tapering and finished in polished nickel, making them 
veritable mirrors. As the image of the source of light spreads more 
or less beyond the opening of the front lens upon this mirror surface 
the dazzling light is reflected into the face of the operator, and also 
more or less around the room. The operator is likely to be so 
blinded by the reflections that he cannot see to focus properly. 



371a. When necessary, a person can give polished surfaces a dull finish 
himself. A camel's hair artist's brush should be employed for the finer work. 
For the dull finish, dead-black japalac thinned somewhat with xylene (xylol of 
the Germans) toluene or turpentine answers well. 

Dull black may be prepared by adding to thin shellac varnish plenty of good, 
dry lamp-black. After thorough shaking, this should be filtered through 
gauze to take out any coarse particles. If the shellac is too thick the resulting 
finish is more or less shiny, but if the proper mixture is used the surface will 
be very dull, but not so smooth as the japalac. 

As the black surface wears off by use, the bright surfaces underneath are 
exposed, and occasionally one should go over the apparatus and reblacken all 
bright spots. 



246 BLACK APPARATUS FOR MICRO-PROJECTION [Cn. IX 

The light scattered in the room is liable also, if the room is finished 
in a light tint, to diminish the brilliancy of the screen image by 
lessening the contrast. 

To avoid the troubles just considered, the objective should have 
a perforated hood over its front. The perforation should be of the 
diameter of the front lens. The free surface of the hood over the 
front of the objective should be perfectly flat, and should be finshed 
in dull black (fig. 139-140). Such a hood is also of the greatest 
use in enabling one to center the light ( 375, 372a). 





A B 

FIG. 140. END VIEW OF A HOODED OBJECTIVE SHOWING THE LIGHT 
CENTERED AND OFF CENTER. 

In A the image of the crater is directly over the opening in the hood and 
therefore gives the greatest light for projection. 

In B the crater image is at the right and only a small amount of light enters 
the objective. 

In both A and B the negative or lower carbon is shown by cross lines. It is 
above owing to the inverting action of the condenser. 

373. Light shield beyond the objective. There should be a 
flat or concave shield beyond the objective to prevent any stray 
light reaching the screen from the apparatus except what passes 
through the objective (see fig. 133, 138). 

CENTERING THE PARTS OF THE PROJECTION MICROSCOPE ON ONE 
LONGITUDINAL Axis 

374. For micro-projection it is absolutely necessary that all 
the parts or elements should be on one straight longitudinal axis 
like beads on a rod. With the large lenses used in magic lantern 

372a. If one docs not have the metal-hooded objectives (fig. 139), 
ordinary, nickel-plated objectives can be greatly improved by painting the 
bright surfaces with dull black ( 3/ia). The objectionable reflections can 
also be prevented by tying black velvet or blackened asbestos paper around the 
objectives. 



CH. IX] CENTERING FOR MICRO-PROJECTION 247 

objectives a slight variation from perfect alignment would do no 
particular harm, but the lenses arc so small in micro-projection 
objectives that a very slight displacement from the axis would 
throw the light outside the objective and spoil the projection. 

The fundamental principles and precise directions for centering 
projection apparatus are given in Ch. I. 51-58. 

375. Final centering of the projection objective. After the 
lamp and condenser are centered as nearly as possible and are at the 
right distance apart ( 55, 56, 376), move the stage up toward the 
condenser so that there is plenty of room between it and the objec- 
tive. Use some dust or smoke to find where the cone of light from 
the condenser comes to a focus (fig. 132, 323). 

Now move the microscope on its mounting toward the condenser. 
If the objective is centered, then the point of light at the focus will 
enter the front lens through the hole in the objective hood (fig. 140). 
If it is not centered then it will appear at one side or even entirely 
outside the objective. Use the fine adjusting screws of the arc 
lamp and change the position of the image of the crater sufficiently 
to direct the cone of light into the front lens of the objective. In 
case the objective is greatly out of center it may be found necessary 
to change the position of the entire microscope. 

376. Distance of the objective from the condenser. The 

objective should be at a distance which will bring the crossing point 
of the rays in the cone from the condenser within the objective, as 
for the magic lantern objective (fig. 122). As the center of the 
objective is but slightly beyond the front lens, the following method 
has been found to give excellent results. The objective is drawn 
up toward the condenser until the image of the crater is shown 
within the opening upon the black hood in front of the objective 
(fig. 140). As the image is inverted the lower or negative carbon 
will appear above in the image. If now the stage with a specimen 
is moved up toward the objective until the microscopic object on 
the stage is in focus, the image on the screen will be very brilliant. 
One should make slight adjustments toward and away from the 
condenser to get the most brilliant image. It will be found that 



248 CENTERING FOR MICRO-PROJECTION [Cn. IX 

the greatest brilliancy is when there is a slight yellowish tinge to the 
light. It will be pure white if one moves the stage and objective 
slightly nearer the condenser, but it will not be so brilliant. Guid- 
ing marks should be made on the apparatus at the best position 
for the different objectives used (fig. 133, 138). 

377. Table of Can die-Power and Current with Direct Current 
Arc and Right-Angled Carbons : 

Size of Carbons Amperes Candle-Power 

6 mm. 2 200 

6 mm. 3 400 

6 mm. 4 650 

6 mm. 5 goo 

8 mm. 7.5 1.500 

ii mm. 10 2,200 

1 1 mm. 12.5 2,900 

ii mm. 15 3 ,700 

1 1 mm. 17.5 4>5o 

13 mm. 20 5.400 

13 mm. 25 7.500 

15 mm. 30 9.500 

378. Increase in size of the crater with increase of amperage. 

As the size of the crater and hence its image increases with the 
increased amperage, the gain for actual micro-projection is not so 
great as would appear, for the larger crater image will be larger 
than the lenses of objectives of high power, hence, much light is 
wasted (fig. 141). 

The heating is also much increased by the higher amperage. 
It has been found by experience with everything in the best possible 
condition that 12 amperes is sufficient for most micro-projection. 
A current above 20 amperes is a pure waste, as well as a source of 
danger to the specimens and apparatus by overheating. The light 
given by 10 amperes properly utilized yields far better results than 
that from 20 amperes only partly utilized. For the candle-power 
with different amperages see 377. 



CH. IXl 



USE OF PROJECTION MICROSCOPE 



249 



USE OF THE PROJECTION MICROSCOPE 

379. Objectives in a revolving nose-piece. For most projec- 
tion a battery of three objectives would be sufficient. These 
should be: (i) a low power objective to show entire specimens 
(one of 40 to 50 mm. focus is good) ; (2) an intermediate objective 
of 16 to 18 mm. focus; and (3) a high power, that is, one of 10 to 4 
mm. equivalent focus ( 355). 



10 



20 




FIG. 141. SIDE AND FRONT VIEWS OF THE CRATER AND CARBONS 

BURNING WITH 10 AND WITH 20 AMPERES OF DIRECT 

CURRENT (Natural size). 

This picture is to show the increase in size of the crater with the larger cur- 
rent. (See also fig. 292-293). 

(In making the photographs, the lamp was burning with the amperage indi- 
cated, and an instantaneous exposure was made with a diaphragm of F/32. 
The current was then turned off and the carbons exposed 90 seconds with a 
diaphragm of F/8. This brought out the carbons, and gives the appearance 
gained by the eye when suitably screened and looking at the burning lamp.) 



250 USE OF PROJECTION MICROSCOPE [Cn. IX 

The three objectives selected should be in a revolving nose-piece 
(fig. 142) so that one can pass quickly from one power to another. 
The lecturer and operator must always keep in mind that for an 
audience giving their entire attention, a delay of even a quarter of 
a minute seems a very long time, hence every precaution should be 
taken to avoid delays. 

380. Preparation of the carbons for an exhibition. The 

carbons supplied for projection are soft-cored, and sharpened 
somewhat like a lead pencil. This end form is unlike that assumed 




FIG. 142. TRIPLE NOSE-PIECE OR REVOLVER FOR QUICKLY CHANGING 

OBJECTIVES. 

(From the Catalogue of Viogtldnder und Sohn). 

in the actual use of the carbons (fig. 141), and until the carbons 
have burned for some time, one will not get the best light from 
them. Hence it is wise to get the carbons formed by burning them 
in the lamp for five minutes or so before using them for a lecture or 
an exhibition. 

Soft-cored carbons arc a necessity for micro-projection, for the 
crater remains more uniform and it does not wander around the 
end of the carbons and thus get out of line of the general axis so 
frequently as would be the case with solid carbons ( 38oa). 

380a. Cored and solid carbons. Some workers with the projection 
microscope use a large, cored carbon above (i.e., for the positive) and a solid 
carbon for the negative one. For example, in a projection outfit from Zciss 
the upper carbon was 19 mm. in diameter and soft-cored. The lower omega- 



CH. IX] USE OF PROJECTION MICROSCOPE 251 

381. Screen image of the carbons. One of the good ways of 
learning to get the carbons in the correct relative position is to 
study their image on the screen. For this use an objective of 
50 or 100 mm. focus. By moving the objective somewhat 
beyond the focus of the condenser an image of the burning car- 
bons will be projected on the screen and one can tell the exact 
appearance of the crater and the relative position of the car- 
bons. The glowing upper carbon ought to show the crater 
well and appear to face directly toward the observer. As this is 
an image of the real image of the carbons formed by the condenser 
the screen image will appear right side up. If the negative or lower 
carbon is not in the correct position it will shade the image (see 
fig- 24, 25). 

382. Centering the light and getting the objective at the 
correct distance from the condenser for an exhibition. In using 
any of the objectives on the revolving nose-piece it is always to be 
kept in mind that the centering is most easily accomplished by 
drawing the objective toward the condenser until the image of the 
crater and the tip of the negative carbon appear in the opening and 
upon the objective hood (fig. 140). 

Now if this image is not so that the brightest part is over the 
opening in the objective hood, use the fine adjustment of the arc 
lamp and get the image of the crater directly in the opening. The 
screen image will then be evenly and brilliantly lighted. In case 
one side is more brilliantly illuminated than the other, one can 
make the illumination even by the fine adjustments of the arc lamp 
(fig. 3, 146). 

One can sometimes improve the illumination slightly by looking 
at the screen image and moving the microscope slightly nearer or 
farther from the condenser, but as a rule, when the image of the 

tivc carbon was 13 mm. in diameter and solid. In Ewon's lamp the upper or 
positive carbon is eored and 18 mm. in diameter; the lower carbon is 12 mm. in 
diameter and solid. 

Experience leads us to recommend cored carbons below as well as above. 
For the size of carbons for different amperages see 377, 753a. 

For alternating current both carbons are of the same size, and most workers 
recommend that thev be cored. 



252 USE OF PROJECTION MICROSCOPE [Cn. IX 

crater and the negative carbon arc most sharply defined on the 
objective hood the light on the screen will be the best attainable. 
Occasionally, during an exhibition, it will be necessary to use the 
fine adjustments on the arc lamp (fig. 146) to get the crater back in 
exact alignment as the crater changes position slightly on the wear- 
ing away of the carbons. As the carbons sometimes wear away 
unevenly it is necessary to have a mechanism by which one carbon 
can be moved without affecting the other, otherwise there would 
result some one of the malpositions shown in fig. 24, 25. 

383. Specimens for projection. The specimens giving the 
best images with the projection microscope are those which are best 
for ordinary observation, that is those with the most definite out- 
lines and sharpest details. They must, of course, be more or less 
transparent. For staining, any color which gives definite details 
can be used, but one must remember that the red colors are trans- 
parent to the longer, visible waves of light and hence red-colored 
objects can remain on exhibition much longer than hematoxylin, 
osmic acid or other dark stained objects which are more opaque to 
the long waves in the red end of the spectrum (fig. 307). 

No matter how large the water-cell or the cooling stage, a thick, 
darkly stained specimen will be spoiled after a time by the trans- 
formation of the absorbed light into heat ( 852). 

384. Masks for microscopic slides. The light used in pro- 
jection is of necessity so brilliant that the scattered light from the 
microscopic glass slide is very liable to dazzle the eyes of the 
operator when he looks at the slide in arranging it for the projection 
of the object or objects thereon. If one has a series for example, it 
is very difficult to select with ease and certainty just the sections 
that are to be shown with this scattered light in the eyes. It must 
always be remembered, too, that a very short time seems long to a 
waiting audience; and that it lessens their confidence in the lec- 
turer to have too much blundering in showing the specimens he 
wishes them to see. 

All this difficult}- can be easily avoided by properly masking the 
preparations to be shown (fig. 143, 148). 



CH. IX] 



USE OF PROJECTION MICROSCOPE 



253 



385. Kind and color of paper for the masks. The best paper 
to use is one that allows only a moderate amount of light to pass, 
and that cuts out the green-blue end of the spectrum. 

The color found best for this is an aqueous solution of the 
microscopic stain known as "Orange G." For the quality of 
paper, a white linen bond paper of moderate weight is used. It is 
stained by soaking it a few minutes (10-30) in a saturated aqueous 
solution of the "Orange G." It is then hung up to dry. 




FIG. 143. SLIDE OF SERIAL SECTIONS WITH MASK. 

The sections to be demonstrated are left uncovered. 
Sus (Sus scrofa, the pig). 

Ser. ii This shows that the slide is from the 1 1 th series of pig embryos. 
si 60. The 6oth slide of series 1 1 . 

Sec ij/J- This indicates that the sections of this embryo were cut 15 microns 

(.015 mm., .00058 in.) thick. ^i" 

i goo The year in which the series was prepared. fit* 

ii 60 At the left; series 1 1, slide 60. K* 

& 
Paper thus colored allows a moderate amount of light to pass, 

and allows practically all of the long waves of reel and infra-red to 
pass, so that it will not burn very quickly in the focus of the con- 
denser. If black paper were used it would burn almost instantly 
in the focus. Of the many yellows and oranges tried for masks the 
"Orange G" proved most satisfactory. 

386. How to employ the masks. The paper is cut of the 
right size for the slide and then square or round holes arc made in 
it to give a clear field for the different objects to be shown on the 
slide, then it is pasted on the cover-glass (fig. 143). It is put on 



254 L"SE OF PROJECTION MICROSCOPE [Cn. IX 

the cover-glass and not on the slide for the reason that if it were put 
on the slide it would almost entirely overcome the good effect of the 
stage cooling cell, as it would hold the slide away from the glass 
surface, so that the heat could not be carried off by conduction. 
If it is on the cover-glass, the slide can then rest directly against 
the stage water-cell. 

If one ever wishes to remove the mask it is easily done by putting 
a piece of wet blotting paper upon it till thoroughly softened. It 
can then be peeled off, and the cover-glass cleaned with a wet cloth. 

387. Field of view in the screen image. Except with objec- 
tives corrected in the manner of photographic objectives the screen 
image will not be equally sharp over the entire field where the large 
tube and where no tube is used (fig. 138, 145). To obviate this, 
oculars may be used, or iris diaphragms to cut off the outer margin 
which is not sharp. This margin also shows color from the 
chromatic aberration of the condenser. But demonstrations in 
histology and embryology, at least, depend largely in their effec- 
tiveness upon the relations of parts shown in a large field. The 
part to be shown with greatest distinctness is brought into the 
middle of the field as with ordinary microscopic observation. 

The importance of a large field in which the relation of parts can 
be shown, can be illustrated by a simple experiment. For example, 
let a well known friend cover his face with a mask having only eye- 
holes, or with a hole to show a part of the cheek or forehead. It 
would be hard to recognize him from that limited view alone. 

388. Objectives needed for different sizes of field. In fig. 
1 44 there is given a graphic representation of different sizes of field 
or object which one might wish to project, and the objective or 
objectives with which it can be done. It will be seen that the 
larger the field the longer must be the focus of the projection 
objective. In this figure it is assumed that no ocular is used and 
that the field is not restricted by the tube of the microscope, hence 
for the largest fields the objective must be mounted in a shield 
without tube (fig. 138). In fig. 137 is shown how the field may be 
cut down by using microscope tubes of different diameter. See 
also the table of magnification and field ( 391). 



CH. IX] 



USE OF PROJECTION MICROSCOPE 



255 



1 mm. Field 
Objectives {JSS: 



O 

2-1.5 mm. Field 
Objectives i!::; 

O 

2.5 mm. Ffeld 
Objectives {'? 5 mT 



O 



5 mm. Field 
Objectives { IS mT 



25-20 mm. Field 
Objectives {35 




FIG. 144. SIZES OF FIELD AND OBJECTIVES NECESSARY TO PROJECT 
OBJECTS OF THESE SIZES. 



256 



USE OF PROJECTION MICROSCOPE 



[CH. IX 



389. Sharpness of the screen image. It is a mistake to think 
that it is necessary that the screen image should be photograph- 
ically sharp. As well said by Lewis Wright, p. 191: "A certain 
breadth or coarseness of line is a positive advantage in the image to 
be viewed many feet [meters] away." Of course, the image should 
be focused as sharply as possible, but a line or structure that 
appears perfectly distinct at a considerable distance may appear 
indistinct when the observer is close to the screen. If the operator 
is at a considerable distance (15 to 20 meters, 50 to 65 ft.) from the 
screen, he will find good opera-glasses a help in getting the screen 
images properly focused. 




FIG. 144:1. DIAGRAM TO Snow THE POSITIONS OF THE .SAME OBJECT AND 

THE SIZE OF THE SCREEN IMAGE FOR OBJECTIVES OF =JO, 2O AND IO MM. 

Focus. 

In this figure it is assumed that the object in each case is practically at the 
principal focal distance from the objective and that, the screen distance is the 
same for all. As the size of the image varies inversely with the distance of the 
object from the objective it is seen that the screen must be larger for an 
objective of short than for one of long focus in accordance with the general 
law of the relative size of the object and image ( 392a). 



CH. IX] 



MAGNIFICATION IN MICRO-PROJECTION 



257 



Any one can get a pretty correct idea of the screen image and the 
details visible at different distances by putting the first page of a 
newspaper up in a well lighted place and then moving back from 
it. Close up, the ordinary print can be read, farther away the 
ordinary headlines, and still farther the title of the newspaper or 
some gigantic headline. Meantime the ordinary print and the 
ordinary headlines have merged into a gray haze. 

390. Position of the object on the stage. For many micro- 
scopic specimens it makes no difference how the specimen is placed 
upon the stage, except that for high powers the cover-glass must 
be next the objective. If a specimen must have a given part, end 
or border at the top in the screen image, then with an objective only 
or an objective and an amplifier, the object must be put on the 
stage so that the part is down which is to appear at the top in the 
screen image. With an objective and ocular the object should be 
placed on the stage as the image is to appear on the screen. For 
getting the screen image exactly like the object see 36, 512. 

391. Magnification and Screen Image of Various Objectives 
as Found by Actual Measurement (see 39 ia). 





SM. (i6ft.) 


7.5 M. (25 ft.) 


10 M. (33ft.) 




Screen Distance 


Screen Distance 


Screen Distance 


Objective 


Field of 
Objective 


Magni- 
fication 


Screen 
Image of 
Field 


Magni- 
fication 


Screen 
Image of 
Field 


Magni- 
fication 


Screen 
Image of 
Field 






No TUBE (Fig. 138) 






125 mm. 


55 mm. 


39 


2.25 M. 61 


3.56 M. 


80.6 


4.50 M. 


IOO 


5<> 


48 


2.70 74 


3.16 


98.4 


4.20 


7 


5 


72 


3-50 


107 


5.10 


143 


7.10 


60 


42 


85 


3-15 


123 


3-7" 


1 68 


5-20 


50 


38 


101 


3-30 


147 


4-50 


202 


6.10 


35 


26 


142 


3-50 


2IO 


5.10 


285 


7.60 


3 


20 


167 


3-30 


250 


4.90 


330 


6.70 


20 


II 


253 


2.85 370 


4.20 


495 


5-40 



258 MAGNIFICATION IN MICRO-PROJECTION 

LARGE TUBE (Fig. 121) 



[CH. IX 





5 M. (i6ft.) 


7.5 M. (25 ft.) 


10 M. (33ft.) 




Screen D stance 


Screen distance 


Scieen Distance 


Objective 


Field of 
Objective 


Magni- 
fication 


Screen 
Image of 
Field 


Screen 
Magni- Irnage of 
fication Field 


Magni- 
fication 

I 


Sc:een 
Image of 
Field 






No TUBE (Fig. 138) 






70 mm. 


31 mm. 


72 1.85 M. 


107 


3-34 M. 


143 


4.26 M. 


60 ' 


25 " 


85 


2.IO 


127 


3-oo " 


: 1 68 


4.10 " 


50 " 


22 


101 


2-30 " 155 


3-50 " 


205 


4-50 " 


35 ' 


14 


142 


2-35 " 


218 


3.00 " 


285 


4-45 " 


30 ' 


12 


167 


2.OO " 


250 


3-35 " 


1 330 


4.10 " 


20 ' 


8 


253 


2.10 " 


380 


3.00 " 


1 500 


4.10 " 


16 " 


5-75 " 


322 


i-75 


488 


2.85 " 


! 650 


3-73 " 


12.5" 


4-5 


385 


i. 80 " 


590 


2.65 " 


750 


3-38 " 


10 " 


3-7 


454 


75 " 


7OO 


2-59 " 


900 


3-33 " 


8 " 


2-5 


640 


.80 " 


940 


2-35 " 


1280 


3-20 " 


6 " 


2.18 ' 


760 


.80 " 


I 1 2O 


2.44 " 


1460 


3.18 " 


4 ' 


1.42 ' 


1080 


.70 " 


I6OO 


2.27 " 


!2i8o 


3.10 " 


2 ' 


0.42 ' 


2600 


.10 " 


3820 


1.70 " 


5080 2.OO " 



MAGNIFICATION AND SCREEN IMAGE OF VARIOUS OBJECTIVES AS FOUND 
BY ACTUAL MEASUREMENT. 

First is given the magnification of the objective only, using the large tube 
of the micioscope (fig. 121) ; then are given the magnification, etc., with am- 
plifiers (fig. 126) and wuh oculars. With the latter the draw-tube is in place 
(fig. 147, 172, 3913). 





5 M. 


d6ft.) 


7.5 M. (25 ft.) 


10 M. (33 ft.) 




Screen 


Distance 


Screen Distance 


Screen Distance 


Objective 


Ampli- 
fier 


Micro- 
Ocular scope 
Field 


Magni- 
fication 


Screen 
Image of 
Field 


Magni- 
fication 


Screen 
Image of 
Field 


Magni- 
fication 


Screen 
Image of 
Field 


1 6 mm. 




5.75 mm. 


322 


1-75 M. 


488 


2.85M. 


650 


3-78 M. 


" " - 5d 


4-2 


550 


2.30 " 


820 


3.56 " 


I IOO 


4.85 " 


" " -loci 


4-2 


800 


3-20 " 


I 1 6() 


5.00 " 


1650 


6.89 " 


11 ii 




Proj. X2 1.45 ' 


640 


95 " 


950 


1.40 " 


1320 


I.9I " 


ii ii 


X4 1.32 ' 


I 170 


1.70 " 


1900 


2.65 " 


2610 


3-50 " 


ii ii 


Comp. x2 2.35 ' 


650 


i-55 " 


IOOO 


2.35 " 


1350 3-10 " 


ii ii 


" X4 i. 60 " 


1320 


2-35 " 


2000 


3-40 " 


2880 


4.60 " 


ii 11 




Huyg. X4 i. 60 " 


1310 


2.25 " i 2250 


3-35 " 


2700 14.45 " 


12.5 mm. 


4.5 mm. 


385 


i.8oM. 


590 [2.67M. 


750 3.40 M. 


11 ii 


- 5<1 3-5 


650 


2.30 " 


990 


3.60 " 


1280 4.85 " 


ii ii 


-iod 3.5 910 


3-3<> " 


1430 


5-20 " 


1900 6.50 " 


ii 11 


Proj. X2 1.25 ' 730 


95 " 


1075 


1.35 " 


1450 1.86 " 


ii 11 


X4 1.25 ' 1300 


.70 " 


2OOO 


2-45 " 


2700 3.30 " 


ii ii 


Comp. x2 2.00 " 750 


5 " 


1150 


2.30 " 


1550 3.05 " 


ii 11 


" X4 1.50 " ! 1520 


2-35 " 


2350 


3-50 " 


3200 4.75 " 


ii 11 


Huvg. x4 i .40 " , 1520 


2.2.S " 


1 2350 


3-37 " 


3150 4-45 " 



CH. IX] 



MAGNIFICATION IN MICRO-PROJECTION 



259 





5 M. (i6ft.) 


7.5 M. (25 ft. 


10 M. (33ft.) 




Screen Distance 


Screen Distance 


Screen Distanca 


Objective 


Ampli- 
fier 


Ocular 


Micro- 
scope 
Field 


Magni- 
fication 


Sci een 
Image of 
Field 


Magni- 
fication 


Screen 
Image of 
Field 


Magni- 
fication 


Screen 
Image of 
Field 


10 mm. 






3.70 mm. 


454 


1.75 M. 


7OO 


2.52 M. 


9OO 


3.65 M. 


" " 


- 5d 




2.70 " 


766 


2. 2O " 


HIS 


3-40 " 


1540 


4.70 ' 


" " 


-xod 




2.70 " 


IIOO 


3.20 " 


1630 


4-50 " 


2225 


6.50 ' 


" " 




Proj. X2 


1.05 ' 


870 


95 " 


1300 


1.40 " 


1750 


1.90 ' 


" " 




" X4 


1.05 ' 


1600 


1.70 " 


2330 


2.40 " 


3100 


3-30 ' 


" " 




Comp. x2 


1.70 ' 


870 


1.50 " 


1330 


2.25 " 


1780 


3-05 ' 


" " 




" M 


1-25 ' 


1820 


2-35 " 


2720 


3-50 " 


3680 


4.70 ' 






Huyg. X4 


1-25 ' 


1800 


2.25 " 


2700 


3-30 " 


3680 


4-55 ' 


8 mm. 






2.5 mm. 


640 


i.SoM. 


940 


2.55 M. 


1280 


3-31 M. 


" " 


- 5d 




1. 80 " 


II2O 


2.30 " 


1650 


3-24 " 


2250 


4-36 " 


" " 


-lod 




1. 80 ' 


I6OO 


3.20 " 


2430 


4-35 " 


3250 


6.00 " 


" " 




Proj. x2 


0.7 


1280 


.90 " 


1960 


1.40 " 


2610 


1.89 " 


" " 




" X4 


0.7 


2360 


1.70 " 


3521 


2-35 " 


4820 


3-35 " 


" " 




Comp. x2 


I.IO ' 


1380 


1.50 " 


2030 


2.25 " 


2770 


3.18 " 


" " 




*4 


0.82 " 


2750 


2.30 " 


4I2O 


3.60 " 


5560 


4.90 " 






Huyg. X4 


0.78 " 


2750 


2.25 " 


4050 


3-35 " 


5560 


4-50 " 


6 mm. 






2.18 mm. 


760 


i.SoM. 


1 1 20 


2.50 M. 


1460 


3.50 M. 


" " 


- 5d 




1.7 " 


I27O 


2.25 " 


1950 


3-30 " 


2570 


4-5 " 


" " 


-lod 




i-7 


I8 5 


3-25 " 


2760 


4-95 " 


3700 


6.50 " 


" " 




Proj. x2 


0.60 ' 


1500 


95 " 


225O 


1.41 " 


3100 


1.91 " 


" " 




" *4 


0.60 " 


2720 


1.65 " 


42OO 


2-55 " 


i 5700 


3-37 " 


" " 




Comp. x2 


0-93 " 


1550 


1.50 " 


24OO 


2.30 " 


3200 


3-05 " 


ii 11 




\4 0.70 " 


3120 


2-35 " 


4800 


3-55 " 


6500 


4-75 " 






Huyg. X4 


0.67 


3100 


2.25 " 


4700 


3-37 " 


6500 


4-50 " 


4 mm. 




1.42 mm. 


IO8O 


1.70 M. 


I6OO 


2.40 M. 


2180 


3 .ioM. 


11 11 


- 5d 




1.05 " 


I9IO 


2.30 " 


272O 


3.20 " 


3800 


4-30 " 


ii ii 


-lod 




1.05 ' 


2750 


3-25 " 


4l6O 


4-50 " 


5650 


6.50 " 


ti ii 




Proj. x2 


0.40 ' 


2250 


.92 " 


3460 


1.40 " 


4500 


1.90 " 


ii ii 




" *4 


0.40 " 


4I2O 


1.65 " 


6800 


2-75 " 


9OOO 


3.60 " 


ii it 




Comp. x2 


0.62 " 


2350 


1.50 " 


3500 


2.28 " 


4830 


3.10 " 


ii ii 




" *4 


0.47 " 


4820 


2-37 " 


72OO 


3.60 " 


9800 


4.80 " 






Huyg. X4 


0-45 ' 


4770 


2.25 " 


7IOO 


3-37 " 


9820 


4-50 " 


2 mm. 






0.42 mm. 


26OO 


i.ioM. 


3820 


1.70 M. 


5080 


2.OO M. 


" " 


- 5d 




0.42 " 


4440 


i-95 " 


6560 


2.85 " 


8900 


3.65 " 


ii ii 


-lod 




0.42 " 


722O 


2.60 " 


9480 


4-50 " 


12550 


7.00 " 


ii ii 




Proj. x2 


0.185 " 


4940 


o-93 " 


7400 


1.40 " 


10500 


2.37 " 


" " 




" *4 


0.18 " 


9160 


1.67 " 


13800 


2.50 " 


18750 


3.70 " 


ii 




Comp. x2 


0.28 " 


5120 


1.50 " 


6666 


2.28 " 


IO6OO 


3.00 " 


ii u 




X4 


0.215 " 


10500 


2. 2O " 


16150 


3-35 " 


2I25O 


5.00 " 






Huyg. X4 


0.20 " 


10750 


2. 2O " 


15500 


3-45 " 


2IOOO 


5.10 " 



260 



MAGNIFICATION IN MICRO-PROJECTION 



[CH. IX 





2.5 M. (8ft.) 




Screen Distance 


_, . .. i Ampli- 
Objective j fier 


Ocular 


Microscope 
Field 


Magni- 
fication 


Screen 
Image of 












Field 


2 mm. 




0.42 mm. 


1300 


0.56 M. 


11 11 


- 5d 




0.42 ' 


2130 


0.97 " 


" " 


-lod 


0.42 " 


3080 


1.38 " 


U 11 




Proj. x2 0.185 " 


2350 


0.44 " 


" " 




X4 0.18 " 


4400 


0.78 " 


11 11 




Comp. x2 0.28 " 


2440 


0.71 " 


" 




X4 0.215 " 


5000 


I.IO " 


Huyg. X4 0.20 ' 


4960 i i.io " 



391a. In preparing this table the apparatus shown in fig. 121, 138 was 
used. The second element of the condenser giving the cone of light, had a focus 
of 30.3 cm. (8 in.), and the stage was moved up in the light cone (fig. 132) to 
give the largest and brightest field possible for the given objective. No sub- 
stage condenser was used except for the 2 mm. oil immersion. 

A stage micrometer in millimeters, tenths and one-hundredths was used as 
object. The screen image of one or more of the micrometer divisions was 
measured with a metric rule and the magnification obtained by dividing the 
size of the image by the known size of the object. For example: if the 
micrometeris in one-tenth millimeters (o.i mm.) and the screen image of two 
spaces (0.2 mm.) measures 20 centimeters or 200 mm. the magnification of the 
screen image must be 200 divided by 0.2 = 1000. That is, the image is one 
thousand times the size of the object, therefore, the magnification of the pro- 
jection apparatus in that case is 1000. The size of the field of the projection 
apparatus is found by the use of the micrometer as follows: The micrometer 
is arranged on the stage so that the image shows one of the lines on one edge 
of the field (the circle of light). Then one simply counts the spaces to the 
other edge of the field. For example, suppose that it requires 14 of the o.i 
mm. spaces, then the size of the field is 1.4 mm. and an object larger than this 
cannot be projected entire with this objective. 

To get the size of the screen image of this field a tape measure or meter stick 
is used and the diameter of the circle of light on the screen is measured. 

This method of finding the size of the field of the projection apparatus, the 
magnification and the size of the screen image, depends upon direct observation 
anil is applicable to any projection outfit whether an objective only or an objec- 
tive and an amplifier or an objective and an ocular are used (see also 392a). 
The amplifiers used had a free opening of 36 mm. (i y^ in.), and were placed at 
the end of the large tube (fig. 133) at a distance of about 1 1 cm. (4^4 in.) from 
the objective. 



CH. IX] 



MAGNIFICATION IN MICRO-PROJECTION 



261 



392. Magnification and Screen Image of Various Objectives 
as Found by Calculation (see 392a). 





5 M. (i6ft.) 


7.5 M. (2 5 ft.) 


10 M. (33ft.) 




Screen Distance 


Screen Distance 


Screen Distance 


Objective 


Field of 
Obiective 


Magni- 
cation 


Screen 
Image of 
Field 


Magni- 
fication 


Screen 
Image of 
Field 


Magni- 
fication 


Screen 
Image of 
Field 






No TUBE (Fig. 138) 






125 mm. 


90 mm. 


40.0 


3.6oM. 60 


5.40 M. 


80 


7.2oM. 




55 " 




2. 2O " 




3-30 " 




4.40 " 


IOO " 


75 " 


50.0 


3-75 " 


75 


5.62 " 


IOO 


7-50 " 




50 " 


2.50 " 




3-75 " 


5.00 " 


70 " 50 ' 


7i-5 


3-57 


107 


5-35 " 


138 6.90 " 


60 ' 42 ' 


834 


3-50 " 


125 


5-25 " 


166 7.00 " 


50 " 38 ' 


IOO.O 


3.80 " 150 


5-70 " 


200 7.60 " 


35 ' 26 ' 


143-0 


3.72 " 214 


5.56 " 


286 7.43 " 


30 ' 


20 ' 


166.6 


3-34 " 


250 


5.00 " 


333 6.66 " 


20 ' ii ' 


250.0 , 2.75 " 


375 


4-13 " 


500 5.50 " 



LARGE TUBE (Fig. 121) 



70 mm. 


31 mm. 


7i-5 


2.22 M. 


107 


3-34 M. 


138 


4.27 M. 


60 " 


25 " 83.4 


2.08 " 


125 


3-12 " 


1 66 


4.16 " 


50 


22 


IOO.O 


2.20 " 


150 


3-30 " 


200 


4.40 " 


25 


14 


143-0 


2.OO " 


214 3.OO " 


286 


4.00 " 


30 


12 


166.6 


2.0O " 


250 3.00 " 


333 


4.00 " 


25 


8 


250.0 


2.OO " 


375 


3-00 " 


500 


4.00 " 


16 


5-75 " 


312.5 


1.79 " 


468 


2.69 " 


625 


3-59 " 


12.5 " 


4-50" 


400.0 


1. 80 " 


600 


2.70 " 


800 


3.60 " 


10 


3-70" 


500.0 


1.85 " 


750 


2.78 " 


IOOO 


3-70 " 


8 


2.50" 


625.0 


I. 5 6 " 


937 


2-34 " 


1250 


3-12 " 


6 " 


2.18" 


33-3 


1.82 " 


1250 


2.72 " 1666 


3-63 " 


4 


1.42 " 


1250.0 


1.77 " 


1875 


2.66 " 


2500 


3-55 " 


2 " 


0.42 " i 2500.0 


1.05 " 


3750 


i-57 " 


5000 2.69 " 



392a. This table was derived by calculation from the optical law that: 
The size of the image is to the size of the object as the distance of the image 
is to the distance of the object from the center of the projecting lens or objec- 
tive (fig. 209). In each case the objective's principal focus is marked upon it 
by the maker, and the distance of the screen from the objective is known. 
R~ef erring to the diagram (fig. 121) it is seen that the focus of the objective 
represents approximately the distance of the object from the center of the 
objective when the screen distance is relatively great. The focus of the objec- 
tive and the screen distance being known their ratio is easily found. For 
example, with the 20 mm. objective and a 5 meter screen distance, the object 
will be 20 mm. from the center of the objective (fig. 209) and the screen image 
is 5 meters (5000 mm.) distant, then the ratio is 250 to i (5000/20) and it 
follows from the optical law given above, that the magnification in this case 
is 250. 

The field in each case was determined by the use of a stage micrometer as 
\vith39ia. From fig. 209 it is evident that the screen image of the entire 



262 ORDINARY MICROSCOPE FOR PROJECTION [Cn. IX 

MICRO-PROJECTION WITH AN ORDINARY MICROSCOPE 

393. Magic lantern with optical bench and ordinary micro- 
scope. If one has a magic lantern with an optical bench, the 
bellows and lantern-slide objective may be removed and an ordinary 
microscope put in place. The microscope is made horizontal and 
firmly clamped to a suitable block (fig. 145, 187). This block 
should be furnished with cleats or grooved so that it will slide on 
the rods or guides of the magic lantern, and be of sufficient height 
to put the objective and tube of the microscope in the optic axis. 
The mirror and the substagc condenser may be removed or turned 
aside and the object lighted by the cone directly from the large 
condenser as in fig. 145 or the condenser and ocular may be left 
in place (fig. 187). 



field is magnified, hence to get the size of the screen image, the size of the field 
is multiplied by the magnification of the apparatus in any given case. In the 
case of the 20 mm. objective the entire field measures 8 mm., hence its screen 
image, with a magnification of 250, should be 8 x 250 = 2000 mm. or 2 M. 

If one compares the tables obtained by actual measurement and that 
obtained by calculation it will be seen that they do not exactly agree. This is 
due to two things: first, the rated focus of the objective is only an approxima- 
tion, and second, the measurement of the diameter of the screen image is not 
very exact from the difficulty of deciding just where to begin and where to 
leave off in measuring to get the magnification and for determining the size of 
the field or the screen image of the field. 

The table of calculated values is only for the objective without the use of 
amplifiers or oculars. 

If one knows the magnification of the objective for a given screen distance 
the magnification obtained when using an amplifier or an ocular with the 
objective may be obtained approximately as follows: 

For 5d amplifier multiply the magnification of the objective only, b\ 

For lod amplifier multiply the magnification 

For x2 projection ocular multiply the magnification 

For x4 projection ocular multiply the magnification 

For x2 compensation ocular multiply the magnification 

For x4 compensation ocular multiply the magnification 

For X4 Huygenian ocular multiply the magnification 

As the field of the projection apparatus is cut down by the vise of an amplifier 
or an ocular one must determine the size of the field by the use of a micrometer 
as with the objective alone. The screen image can then be calculated by 
multiplying the observed size of the field by the magnification of the combined 
objective and ocular or amplifier. It will be seen that the objective with an 
ocular x2 or x4 does not give a magnification exactly twice or four times as great 
as the objective alone. The oculars are rated for the ordinary distance of 
distinct vision (254mm., 10 in.) and the relation does not hold strictly for the 
much greater screen distances ( 357a). 




CH. IX] ORDINARY MICROSCOPE FOR PROJECTION 263 




FIG. 145. ORDINARY MICROSCOPE FOR PROJECTION. 

This figure is to show how an ordinary microscope can be used for projection 
if one has an arc lamp and condenser. 

Commencing at the left : 

The supply wires coming to the table switch. 

From the negative pole of the switch one wire proceeds to the negative bind- 
ing post of the arc lamp, i. e., to the one for the lower carbon. 

From the positive pole of the switch extend two wires for the automatic lamp 
of the Bausch & Lomb Optical Co. One wire goes to the binding post of the 
automatic mechanism (the middle post). This means that the automatic 
mechanism receives current which does not go through the rheostat. The 
other wire from the positive pole of the switch goes to the ammeter (A), and 
from the ammeter to the rheostat (R), and from the rheostat to the positive 
binding post for the arc lamp, i. c., for the upper carbon. 

The arc lamp is shown through the metal lamp-house. The lamp-house 
appears transparent as it was left in position during only a part of the exposure. 

Following the lamp-house is the triple condenser and water-cell. 

The microscope is bent over in a horizontal position to bring the axis of the 
objective in line. 

The microscope is clamped to a block which raises it to the right level. 



264 ORDINARY MICROSCOPE FOR PROJECTION [Cn. IX 

As here shown the substage condenser and mirror have been removed, and 
also the draw-tube and ocular (see fig. 147, 192 for the ordinary microscope 
with substage condenser, draw-tube and ocular in position). 

The lamp, condenser and microscope are on independent blocks and can be 
moved to any desired position on the baseboard. 

A The ammeter to indicate the amount of current. 

R Adjustable rheostat. This rheostat is adjustable between 10 and 20 
amperes. The arrow indicates the direction of increase in current. 

5 Adjustable drawing shelf attached to the front legs of the table. In this 
picture the shelf supports the stage of the projection microscope (fig. 121), and 
a box of demonstration specimens. 

The scale of the picture is indicated by the 10 cm. rule just above the table 
drawer at the right. 

If the tube of the microscope is large it is an advantage, but with 
the small tube one can do much. If the ocular is not to be used, 
then it is better to remove the draw-tube so that only the main 
tube remains. One should be sure that the interior of the tube is 
dull black ( 370). 

394. Magic lantern with rods, and an ordinary microscope. 

If the magic lantern has the simple construction with rods and feet 
(fig. 32, 33, 36) an ordinary microscope can be used with it as 
follows: Remove the rods, bellows and projection objective, and 
support the arc lamp and the condenser on a block which will lift 
them high enough so that the microscope in a horizontal position 
will be in the optic axis. Place all on a baseboard with guides 
(fig. 146). Clamp the microscope to a suitable block with grooves 
or cleats to enable one to move the block accurately along the 
guides. When properly centered this form of apparatus w^orks 
well. 

394a. For a water-cell one of the plane-sided glass boxes found on the 
market can be used, or a cell can be prepared in the laboratory as follows: 
Select some good plane and clear glass. For the ends of the box make two 
strips about 2^2 cm. (i in.) wide and about 10 cm. (4 in.) long. For the sides 
use two sheets about 10 cm. (4 in.) wide and II cm. (4^2 in.) long; and for 
the bottom a rather thick sheet or strip about u cm. (4^2 in.) long and 3 cm. 
(i '4 in.) wide. The pieces of glass are then put together by placing the bottom 
on a level table and the other pieces in position and held in place by a string 
or by narrow strips of gummed paper. 

The joints are then gone over carefully with an artist's brush dipped in 
Ripolin white paint or Valspar varnish. Each coat should be allowed to dry 
thoroughly before adding the next, that is, for two to five days. Finally one 
can add water to see if the joints are all tight. It" not, dry the glass box am', 
then add more of the Ripolin paint or Valspar varnish. 



CH. IX] ORDINARY MICROSCOPE FOR PROJECTION 



265 




FIG. 146. USE OF THE SIMPLE 
MAGIC LANTERN CONDENSER 
AND LAMP AND AN ORDINARY 
MICROSCOPE FOR PROJECTION. 

This is a magic lantern with 
iron legs and rods for the support 
and guidance of the parts (fig. 
33). The slide-carrier bellows 
and lantern objective with the 
guide rods have been removed, 
leaving only the condenser, arc 
lamp and lamp-house. The short 
tubes for the lamp are supported 
at the left by the ordinary legs 
of the apparatus. In front a 
support of wood is used when 
necessary . As the whole lamp and 
condenser would be too low for 
the axis of the microscope it is 
raised on a block (BlockJ to the 
proper height. There is a base- 
board on which all the apparatus 
is placed, and at the left there is 
a track made of rods or tubes as 
in fig. 158, 159 on which the block 
supporting the microscope can 
be moved back and forth in line 
of the axis. For a water-cell, a 
glass box made as described in 
394a is set on a block in the 
path of the cone from the con- 
denser. 

Commencing at the left : 
Arc lamp The hand-feed, 
right-angle carbon arc lamp. 
5. s Set screws. 

This is also an excellent meth- 
od of making small glass boxes for 
experimental work where water is 
the liquid medium. Such boxes 
also have been used continuously 
for months for observing the 
growth of aquatic plants. If one 
side is made of cover-glass, then 
high powers of the microscope 
can be used to study the growth 
on the inside face of the cover- 
glass. 

We are indebted to Prof. Romyn 
Hitchcock for the method of mak- 
ing water-cells by the aid of 
Ripolin paint. 



266 ORDINARY MICROSCOPE FOR PROJECTION [Cn. IX 

F. S. Feeding screws for the carbons. 

V. A. Vertical fine adjustment for centering the crater. 

L. A. Lateral fine adjustment for centering the crater. 

W t Supply wire to the upper carbon. 

W a Supply wire from the lower carbon through the rheostat (K). 

R Rheostat in the wire from the lower carbon. 

Rods The short tubes or rods supporting the lamp. 

Z,,, L 2 The left and right supports or legs of the lamp-rods. 

Block^ The block on the baseboard to elevate the arc lamp and condenser 
to the axis of the microscope. 

Lamp-House The metal enclosure of the arc lamp. 

V Ventilator of the lamp-house. 

Condenser The two-lens condenser. It is supported by the front end of the 
lamp-house. 

/, 2 The two plano-convex lenses forming the condenser. 

Water-cell The glass vessel with plane sides filled with water and placed in 
the path of the cone of light from the condenser to absorb the radiant heat. 

Microscope An ordinary microscope turned in the horizontal position. The 
draw-tube and ocular have been removed, also the substage condenser. 

Stage The stage of the microscope. 

SS The substage condenser sleeve. The condenser has been removed. 

Axis, Axis The principal optic axis of the condenser and the microscope. 

() Objective. 

// Handle for carrying the microscope. 

c, f Coarse and fine focusing adjustments. 

cl Clamp for holding the microscope to the block. 

FM Foot of the microscope. 

Block 2 The wooden block supporting the water-cell. 

Block 3 The block to which the microscope is clamped. It moves back and 
forth on the track (tr). 

tr The rods on the baseboard serving for a track. 

Base Board The board on which all the apparatus is placed. 

395. Stray light, and a water-cell. For a water-cell, any glass 
vessel with plane sides can be used, and it can be put between the 
condenser and the stage of the microscope instead of between the 
lenses of the condenser as in fig. 4, 167. For cutting off stray 
light one can use a black cardboard shield, or a black disc may be 
perforated and hung on the end of the tube of the microscope 
beyond the focusing mechanism. For bellows between the con- 
denser and stage, use a sheet of asbestos paper. 

396. The directions for using the ordinary microscope in 
projection arc precisely as for the special microscope shown in fig. 
121, and discussed in the first part of this chapter. As there is no 
stage-cooling device one must be careful not to overheat the speci- 
mens. 



CH. IX] ORDINARY MICROSCOPE FOR PROJECTION 267 



I h 




FIG. 147. PROJECTION WITH THE MICROSCOPE IN A VERTICAL POSITION. 

W vSupply wires from the outlet box (fig. 3). 

r Rheostat of the theater-dimmer type. 

t w Wires to the arc lamp from the switch. 

/ a Fine adjustment screws projecting behind the lamp-house. 

h f Hand-feed screws for the carbons of the arc lamp. 

/ h Lamp-house. It is of sheet iron, but was left in position only a part of 
the time, hence it appears transparent. 

g Observation window opposite the crater. 

C Triple condenser with water-cell (fig. 121). 

a a Principal optic axis. The mirror of the microscope reflects the light 
vertically along this axis and through the microscope, then the mirror or prism 
over the ocular reflects it horizontally again. 

m Mirror or prism over the ocular to reflect the light horizontally to the 
screen. 

sh Shield to cut off stray light. 

b Baseboard with track for an optical bench. 

a s Adjustable shelf for drawing. 



268 



PROJECTION OF HORIZONTAL OBJECTS 



[CH. IX 



PROJECTION OF HORIZONTAL OBJECTS 

397. As with the magic lantern, so with the projection micro- 
scope some objects must be left in a horizontal position for projec- 
tion. This requires that the microscope be in a vertical position. 
As the light source is for giving light in a horizontal direction 
(fig. 121), it is necessary to use a mirror or prism to reflect the 
horizontal light upward through the vertical microscope and then 
another mirror or prism above the microscope to reflect the vertical 
light horizontally to the screen. This is shown in fig. 147, 175. 

The ordinary mirror of the microscope serves very well for mak- 
ing the light vertical, but for reflecting it horizontally to the screen 
a prism or a plane mirror silvered on the face is best, as it gives a 
single image, not a double image as would the ordinary glass mirror 
silvered on the back. 

398. Avoidance of stray light with a vertical microscope. 

This is easily accomplished by using a vertical piece of blackened 
cardboard just beyond the microscope as shown in fig. 147. If 
light escapes from the sides one can use pieces of black cardboard 
or asbestos to enclose the microscope more completely. Ordi- 
narily, however, the single black shield beyond the microscope 
will answer. 

399. Sample Objects Suitable for Projection with the Differ- 
ent Objectives (see also 39ga). 

PHOTOGRAPHIC TYPE OF OHJECTIVES (Micro-Planars, etc.) 
Xo Tube (fig. 138) 





Magnification with: 


Object 


Size of Object 


Objective 


5 Meter 
Screen 


7.5 Meter 
Screen 


10 Meter 
Screen 


Brain Section 


55 to 90 mm. 
50 to 75 mm. 

35 to 50 mm. 
25 to 40 turn. 

20 to 35 mm. 


125 mm. 
ioo mm. 

70 mm. 
60 mm. 

50 mm. 

35 mm. 

30 mm. 
20 mm. 


39 
48 

72 
lS 5 

101 

142 

167 
253 


6 1 

74 

107 
123 

147 

210 

250 
370 


80.6 
98.4 

143 
1 68 

202 
285 

330 
4<>.S 


Cerebellum and Brain 
Stem 


Longitudinal Section 
of 40 mm. Embryo . 
Section of Eve 


Section of Injected 
Kidnev 


,V> I lour Chick Entire 
Transaction of Human 
Esophagus . . 


15 to 25 mm. 

10 to 20 mm. 
51011 mm. 


Appendix ( Homo) . . 



CH. IX] OBJECTS FOR MICRO-PROJECTION 269 

Large Tube (fig. 121) 





Magnification with: 


Object 


Size of Object 


Objective 


5 Meter 
Screen 


7.5 Meter 
Screen 


Jo Meter 
Screen 


Pyloric Stomach .... 
Medulla and Olives 
Scalp 


20 to 30 mm. 
15 to 25 mm. 
12 to 22 mm. 
10 to 14 mm. 
8 to 12 mm. 
5 to 8 mm. 


70 mm. 
60 mm. 
50 mm. 
35 mm. 
30 mm. 
20 mm. 


72 

85 
101 

142 
I6 7 

253 


107 
127 

155 
218 

250 
380 


143 
168 
205 
285 
330 
500 


Human Spinal Cord 
Thyroid .... .... 


Adrenal 



ORDINARY MICROSCOPIC OBJECTIVES 
Large Tube (fig. 121) 



Section of Lung or 
Artery 


4 to 5 mm. 


1 6 


mm. 


1,22 


488 


650 


Neural Plate of Am- 
blystoma 


2 to 4 5 mm 


12 


5 mm 


18=; 


SQO 


7 CQ 


Transection of 
Trachea 


2 to 3.7 mm. 


IO 


mm. 


-IS-l 


700 


QOO 


Striated Muscle Longi 
and Transactions 
Nerve Cells in Spinal 
Cord 


i to 2.5 mm. 
i to 2 mm. 


8 
6 


mm. 
mm 


640 

760 


940 
I I2O 


1280 
1460 


Goblet Cells of Intes- 
tine, Mucicarmin 
Stain 
Silvered Endotheliutn 


i to 1.2 mm. 
0.2 to 0.4 mm. 


4 
2 


mm. 
mm. 


1080 
2600 


I6OO 

3820 


2180 
5080 



399a. The preparations listed in the above table are simply examples of 
objects which can be shown entire with the different objectives without oculars. 
In practice any good microscopic preparation and many living things can be 
shown with the projection microscope. 

For the complete understanding of any specimen it is necessary to see it as 
a whole and then by using higher and still higher powers (391) to get views of 
finer and finer details. 

In demonstrating the finer details one can show but a very small specimen or 
a small part of a large specimen. For large specimens it is a great advantage to 
have objectives of different powers on a revolving nose-piece so that it takes 
only a moment to turn from one to the other. If only the large condenser is 
used (fig. 121) the objective remains practically stationary, but the specimen 
must be on a movable stage so that it can be farther from the objective or 
nearer to it depending upon the focal length of the objective (fig. 132). 

If one uses substage condensers the stage remains stationary and a long 
focus substage condenser is used for low powers and a short one for high powers 
and the objective is placed at approximately its focal distance from the object. 

It must be remembered that many living things are soon destroyed by the 
intense light necessary for projection. While the circulation of the blood seems 
an ideal demonstration with the projection microscope it is found in practise to 
be a very poor way to demonstrate it. If this is tried the microscope in a ver- 
tical position (fig. 147) is convenient. The screen distance should not be very 
great (3 to 5 meters, 10 to 16 ft.). In the author's experience the demonstra- 
tion of blood circulation under a microscope is vastly superior to anything that 
can be done with a projection microscope. 



270 EXHIBITION WITH PROJECTION MICROSCOPE [Cn. IX 

CONDUCT OF AN EXHIBITION OR DEMONSTRATION WITH THE 
PROJECTION MICROSCOPE 

400. What is said in Ch. I, 21-40 is entirely applicable to 
the projection microscope by substituting microscopic specimens 
for the lantern slides. Only from the greater difficulty and pre- 
cision demanded in using the projection microscope, it is impera- 
tive that the operator be prepared, hence the greater necessity of 
making certain that everything is in absolute order before the 
lecture begins. 

If any of the projection objectives (i. e., those of 125 to 20 mm. 
focus) have iris diaphragms, open these as widely as possible. 
Never try to project with the iris of the objective partly closed. 




FIG. 148. SI.IDH-TKAV WITH MASKKD PREPARATIONS TO HE USED IN 
PROJECTION. (About Y* size). 

Three series are here represented on different sized slides. 

The seetions to be shown are not eovered with the masking paper. The 
numeral on the side give the number of the series (ser. 90, ser. 17, ser. 15). On 
eaeh slide is also the number of the slide in the series as ser. 15, slide 57, 63, 67, 
etc. 



CH. IX] EXHIBITION WITH PROJECTION MICROSCOPE 271 

An experiment with the iris partly closed and then wide open will 
show the necessity of observing this rule. 

The microscopic slides should be in order and properly masked 
( 384) and marked in some way so that the operator can tell which 
edge up they should be placed on the stage. 

It is also a great advantage to have marked on the microscopic 
specimen the objective or objectives that should be used in pro- 
jecting it to bring out the structural details which it is desired to 
show. 




FIG. 149. SLIDE Box TO HOLD PREPARATIONS FOR DEMONSTRATION. 
(Cut loaned by the Spencer Lens Company). 

For ease in getting hold of the slides to be exhibited, either a 
shallow tray can be used or a slide box (fig. 148, 149). As with 
lantern slides, it is advantageous to have the microscopic specimens 
so placed that they can be grasped easily, and put on the stage as 
desired without hesitation. 

Some teachers, including the senior author, have found it 
advantageous to manage the projection themselves, giving the 
explanations from the position of the lantern. 

The best way to point out the parts in the screen image to be 
especially noted is to have a slender pointer about two meters (six 
feet) long, like the upper two-thirds of a bamboo fishing rod, and 
to hold this out in the beam of light. The shadow appears on the 
screen sharply, and one can point out details with the same clear- 
ness as by using a pointer on the screen. It is easier also, because 
the speaker does not get his eyes dazzled by looking into the light 
beam, as so often happens when standing near the screen in the 
usual lecture position. 

SPECIAL DEMONSTRATIONS WITH HIGH POWERS 
401. Substage condenser in projection. As indicated in 
359 the authors of this book believe that projection for large 
audiences and with low objectives is best accomplished without 



272 HIGH POWER MICRO-PROJECTION [Cn. IX 

substage condensers, and without oculars; but they realize that 
in laboratory work and for some special lectures to small classes it 
is of the highest advantage to be able to show pictures of photo- 
graphic sharpness in all details. For this it is necessary to use, first 
of all, a substage condenser which will give a light cone of sufficient 
aperture for the details; and secondly there must be a proper 
screen, i. e., the screen must be very white and very smooth, but 
not shiny ( 409, 621). White cardboard answers well. Finally 
there must be an ocular used, and the observers must be near 
enough the screen to see the fine points. 




FIG. i5oA. ACHROMATIC, SUBSTAGE CONDENSER WITH 

CENTERING SCREWS. 
(From Zeiss' Catalogue). 

There has been a segment of the condenser cut away to show the construc- 
tion. 

The centering screws (c-s, c-x) enable the operator to get the condenser in 
the optic axis of the microscope. The iris diaphragm for this condenser is 
between the lower and middle combinations, not below the condenser as with 
the Abbe form. 

This form of condenser is especially desirable for projection and for photo- 
micrography. 

The substage condenser for micro-projection must either be of a 
special form to use with the main condenser of the apparatus or 
special means must be employed to utilize the light cone from the 
main condenser when the ordinary substage condenser is used. 

This is because the substage condenser ordinarily used on micro- 
scopes is designed for approximately parallel beams of light, not 
for those markedly converging or diverging. By examining the 
figures of the light cone from the main condenser it will be seen 



CH. IX] 



HIGH POWER MICRO-PROJECTION 



273 



that the cone of light is converging to the focal point and diverging 
beyond that point (fig. 122, 132 and 320-323). If the converging 
cone is used the substage condenser brings it to a focus too soon 
and if the diverging cone, then the substage condenser brings it 
to a focus too far beyond it. 

402. Methods of rendering converging or diverging light 
parallel. There are two principal ways of utilizing the light cone 
from the main condenser. 



Object 





FIG. 1506. ABBE SUBSTAGE CONDENSER SHOWING PARALLEL AND 
CONVERGING INCIDENT LIGHT. 

In this form of condenser the iris diaphragm is below both condenser lenses 
(compare fig. 150). 

With parallel, incident light the condenser focuses the light just above the 
condenser, with converging light the focus is within the upper lens and the 
light is diverging on leaving the upper lens. 

o, o Object. 

Objective The front lens of the projection objective. 

A. Rendering the converging cone of light approximately 
parallel by means of a concave lens. As it is desirable to use all the 
light in the cone, the concave lens is put in the cone where its 
diameter is slightly less than the diameter of the substage con- 
denser, that is about 25 mm. (i in.). The trial glasses used by the 
oculist are excellent for the purpose. A fork with stem is desirable, 
and this is placed in the socket for the mirror stem. This brings 
the fork carrying the spectacle lens near the substage condenser. 
Concave spectacle lenses of 10 to 20 diopters (100 to 50 mm., 4 to 2 
in. focus) have been found excellent. The microscope for projec- 
tion is so placed that the fork carrying the concave lens is about 



274 HIGH POWER MICRO-PROJECTION [Cn. IX 

2^ to 3 cm. (t to i J/2 in.) from the focus of the converging cone. 
The concave lens will render the converging light approximately 
parallel, and this cylinder of light is small enough to enter the 
substage condenser. By a small manipulation of the screw of the 
substage condenser bringing it slightly nearer the specimen or 
slightly farther from it the most brilliant screen image can be pro- 
duced. A slight change in the position of the substage condenser 
often works wonders. 




FIG. 151. RELATION OF THE APERTURE OF THE LIGHT FROM THE COX- 
DENSER TO THE APERTURE OF THE OBJECTIVE. 

(From Nelson, Jour. Roy. Micr. Soc.). 

A The cone of light from the condenser just fills the aperture of the objec- 
tive (B). 

B Back lens of the objective entirely filled with light. 

C The cone of light from the condenser is not great enough to fill the aper- 
ture of the objective (D). 

D Back lens of the objective lighted by the condenser (C). 

The dark ring shows the aperture of the objective not lighted by the con- 
denser. 

B. Rendering the diverging cone of light approximately parallel 
by the use of a convex lens. If a convex lens is placed in the path 
of the diverging cone at its focal distance from the focus of the main 
condenser, the light will be rendered parallel. In order to have a 
cylinder of light of the right size to enter the substage condenser a 
convex lens of the proper focal length and diameter must be used. 
Trial lenses arc excellent. Those of 10 and 20 diopters (100 and 
50 mm., 4 to 2 in. focus) arc excellent for the main condenser with 
a focus of 150 to 200 mm. (6 to 8 in.). The microscope must be 
put in such a position that the trial lens in the fork before the sub- 
stage condenser shall be at its focal distance from the focus of the 
main condenser. The diverging cone of light will be made approxi- 



CH. IX] 



HIGH POWER MICRO-PROJECTION 



275 



mately parallel (fig. 1536), and by slight adjustments of the sub- 
stage condenser brilliant images are produced. 




FIG. 152. MICROSCOPE FOR PROJECTION AND FOR DRAWING. 

W i The negative supply wire from the outlet box (fig. 3). 

W + I The positive supply wire from the outlet box. 

S Double-pole, knife switch. 

W 2 Wire from the switch to the binding post of the lower carbon. 

W + 2 Wire from the knife switch to the rheostat. 

W + 3 Wire from the rheostat to the upper carbon (+ // C). 

ri, r2 The two binding posts of the rheostat. 

Rheostat The controlling device for the current. 

ILC Incandescent lamp cord. 

Inc. Lamp The incandescent lamp with a wire lamp guard. 

This lamp is for use in working about the projection apparatus. It is con- 
nected to the supply wires at their connection with the switch so that the 
incandescent lamp will burn whether the knife switch is open or closed (sec also 
fig. 2, 4). 

Radiant The arc lamp. 

S-\-, S The set screws for the carbons. 

HC, VC The horizontal or upper and the vertical or lower carbons. 

Condenser The triple-lens condenser with water-cell in the parallel beam 
between the two plano-convex lenses. 

Axis, Axis The optic axis of the condenser and the microscope. 

Substage Condenser The achromatic condenser under the stage of the 
microscope. 

P L The concave lens for making parallel the converging light from the 
large condenser before it enters the substage condenser. 

St Stage of the microscope. 

Objective The projection objective. 

Ocular The ocular of the microscope used in projection. 

Af 2 The mirror or prism placed just beyond the ocular when it is desired 
to reflect the light downward. 

Screen Image The image projected upon the white screen by the projection 
microscope. 



2 7 6 



HIGH POWER MICRO-PROJECTION 



[CH. IX 



Bl. R The block carrying the radiant on the optical bench. 

Bl C The block carrying the condenser on the optical bench. 

Bl M The block carrying the microscope on the optical bench. 

Base Board The board bearing the track made of rods and serving as an 
optical bench. 

Projection Table The table supporting the apparatus and holding it at the 
proper height for use. 

The above method refers especially to high powers objectives 
of 2 to 8 mm. equivalent focus. For powers lower than those just 
mentioned one can get better results by the use of a main condenser 
with a second element of 200 to 150 mm. focus and no substage 
condenser, or by adopting the method given below or in 403 . 



Substage 
p [_ Condenser 




FIG. 153. DIAGRAMS TO SHOW METHODS OF PARALLELIZING THE CONE 
OF LIGHT FROM THE MAIN CONDENSER. 

A Method of parallelizing the converging cone of light from the main 
condenser by means of a concave lens within the focus (/). 

B Method of parallelizing the diverging cone of light from the main con- 
denser by means of a convex lens beyond the focus (/). 

Arc Supply The right-angled carbons of the arc lamp. 

L, L 2 The first and second elements of the triple, main condenser. 

Water Cell This is to remove the radiant heat. 

Axis The principal axis on which all the parts are centered. 

/ The principal focus of the second element of the main condenser. 

P. L. Parallelizing lens. Concave in A, Convex in B. 

Substage Condenser This is the first or lowest element of the substage con- 
denser of the achromatic form (fig. i5oA). See also fig. 150 B. for the Abbe 
form of substage condenser. 



CH. IX] 



HIGH POWER MICRO-PROJECTION 



277 



Finally if one uses a main condenser with a focus of 30 or 38 cm. 
(12 to 15 in.) excellent results can be obtained with all powers (16 
to 2 mm.) by so placing the microscope that the converging cone of 
the main condenser shall enter the substage condenser at a point 
where the light cone is of about the diameter of the substage con- 
denser (fig. I54A-B). It may be necessary to raise or lower the 
substage condenser slightly to obtain the most brilliant screen 
image. 

Fair results can also be obtained in this way by using main con- 
densers of 15, 20 and 25 cm. (6, 8, 10 in.) focus, but much more 



Substage 
Condenser 




FIG. 154. 



DIAGRAMS TO SHOW THE POSITION OF THE SUBSTAGE CONDENSER 
WHEN NO PARALLELIZING LENS is USED. 



A The substage condenser is within the focus (/) at a point where the long, 
light cone is of about the same diameter as the substage condenser. 

B The substage condenser is beyond the focus (f) of the long focus main 
condenser, at a point where the diverging cone is of alaout the same diameter as 
the substage condenser. This is the better position for the s.ubstage condenser 
of the ordinary microscope. 

Arc Supply The right-angled carbons of the arc lamp. 

L r L 2 The first and the second elements of the main condenser. 

Water Cell This is to remove the radiant heat. 

Axis The principal axis on which all the parts are centered. 

/ The principal focus of the second element of the main condenser. In both 
cases the focus is long as compared with fig. 153. 

Substage Condenser This is the first or lowest element of the substage con- 
denser. It is of the achromatic type (fig. 150 A). See figure 150 B for 
the Abbe form of substage condenser with parallel and with converging light. 



278 HIGH POWER MICRO-PROJECTION [Ca. IX 

brilliant pictures can be produced by using also a parallelizing lens 
as indicated in 402 A. 

If one has an optic bench apparatus (fig. 121, 158, 159) one can 
get good results with the condensers of all foci by placing the 
microscope so that a diverging cone of light enters the substage 
condenser (fig. 1546). It will then be necessary to lower the 
substage condenser slightly for the higher powers. 

403. Kohler method of using the substage condenser. The 

general principle is shown in fig. 170. The microscope is moved 
toward the main condenser until the focus is at the iris diaphragm. 
One can tell when the main condenser is focused on the iris dia- 
phragm in the same way as that in focusing on the black hood of the 
objective ( 375) viz., by noting when the image of the crater and 
the tip of the lower carbon appear on the iris. After the image is 
focused on the iris diaphragm the iris is opened to admit the cone 
of light, and the substage condenser is raised or lowered slightly to 
get the most brilliant light. As one can see by the diagrams of 
light cones and the plates of the light rays and the light cones, the 
light is diverging beyond the focus so that diverging and not 
parallel light enters the substage condenser. As the condenser 
cannot focus diverging light at the same level that it would focus 
parallel light it may be necessary to lower the substage condenser 
somewhat to get the most brilliant image with high powers. Fur- 
thermore, if a concave lens of 10 to 20 diopters is put in the fork as 
described in 402 A the image will be markedly brighter unless a 
very long focus main condenser is used (fig. 171). (See also Ch. 
XIV, 864). 

404. Aperture of the substage condenser. The purpose of 
the substage condenser in projection, as in direct observation 
with the microscope, is to increase the aperture of the illuminating 
cone. And as it is now one of the fundamental doctrines, that the 
resolution or making visible of minute details depends directly 
upon the aperture of the objective used, naturally as much as 
possible of the aperture of the objective is employed. For this, 
the substage condenser diaphragm should be wide open, so that the 



CH. IX] 



HIGH POWER MICRO-PROJECTION 



279 




FIG. 155. 



THE EFFECT OF USING AN IRIS DIAPHRAGM IN THE CONE OF 
LIGHT FROM THE MAIN CONDENSER. 



The second element of the condenser is shown at the top. The focus of 4 the 
cone of light from the condenser is shown at F, the axis by A . 

B At the right are shown in millimeters, three diameters of the cone of light 
with three different openings of the iris diaphragm (22, 33, 44 mm.) 

C At the left are shown the apertures corresponding with these openings 
in the iris diaphragm (23, 34, 45). The aperture of these openings is also 
shown above the circles. 

One can see by this diagram what an enormous amount of light is lost by 
making the illuminating cone smaller. 



280 HIGH POWER MICRO-PROJECTION [Cn. IX 

entire beam of light from the lamp condenser may enter. Then, 
just as in ordinary observation, one can often make the contrast 
more striking by cutting down the aperture somewhat by closing 
more or less the substage condenser diaphragm. It must not be 
cut down too much, for that will render the image dim and defeat 
the very purpose of the substage condenser. 

As a general statement, much more of the aperture of the 
objective can be used in projection than in ordinary direct observa- 
tion in the microscope. Naturally, objectives of relatively large 
aperture give the more brilliant images (see 855). 

405. Oculars to use in projection. Generally speaking, only 
low powers arc used (x2, X4, x8). The lower the power the more 
brilliant the image. Compensation oculars have been found better 
than the Huygenian. A compensation ocular as high as xi2 gives 
brilliant images for short screen distances. 

One should not forget that the ocular, when used in projection, 
is really a second projection system, and hence the image will be 
erect on the screen (fig. 207). 



404a. Centering the substage condenser. As the substage condenser 
becomes one of the optical elements in projection, its' principal optic axis must 
be centered on the common axis of the entire apparatus. 

It is assumed that the microscope without the substage condenser has been 
properly centered as directed in 374-375. 

To center the substage condenser, use the ocular and objective (x4 ocular, 
8, 10 or 1 6 mm. objective), remove the bellows if present (fig. 133), place a piece 
of white cardboard at about 45 degrees as shown in fig. 1 16, between the large 
condenser and the substage condenser, and light the cardboard well with a 
mazda lamp. This will give the light for the microscope. 

Now put a preparation on the stage and focus the microscope as for ordinary 
observation. Remove the specimen and close the substage iris diaphragm 
nearly up. With a pocket magnifier examine the eye-point or Ramsden's disc 
(fig. 127 E P) beyond the ocular. This disc of light appears as if on the back 
lens of the objective. If the iris is properly made and the substage condenser 
is centered with the objective and ocular, the center of light will appear to lie 
exactly in the middle of the back lens of the objective (fig. 151). If the sub- 
stage is not in the optic axis then the disc of light will appear eccentric; and 
if the substage condenser is markedly off the center the spot of light will make 
a break in the black ring on one side as shown in fig. 30, 1-4. If it is only 
slightly off center, the disc of light will seem to be surrounded by a dark ring 
of unequal width. If the substage condenser is not found to be correctly 
centered, the centering screws (fig. 150) must be used to move it slightly until 
the disc of light is central as shown in fig. 151. 

The Abbe condenser found on most microscope's has no centering screws. 
Tlu' makers center the instrument carefully and fix it in position. If it is 
found badlv out of center it is best to return it to the makers for adjustment. 



CH. IX] 



HIGH POWER MICRO-PROJECTION 



281 



406. Range of objectives to use with a substage condenser. 

Objectives of 16, 12, 10, 8, 6, 4, 3, and 2 mm. equivalent focus 
are used with the substage condenser. For objectives of longer 
focus than 16 the substage condenser of the ordinary form is 
rarely used. Either a special long focus substage condenser is used 
or the ordinary one is turned aside and the cone of light from the 
large condenser used as directed above ( 376). 

407. Change in position of the substage condenser for differ- 
ent objectives and thickness of slides. For the highest powers 




FIG. 156. PROJECTION MICROSCOPE OF ZEISS. 

(From the 4th edition (1899) of Zeiss' catalogue of instruments and appliances 
for Photo- Micrography and Projection). 

This projection apparatus, which in its main features was described in Zeiss 
microscope catalogue No. 28, (1889), and No. 29 (1891), consists of an 
optical bench on which all of the parts needed move separately so that any 
desired arrangement can be made for projection of large objects with low power 
or smaller objects with high powers. 

Commencing at the right: 

/ Arc lamp with inclined carbons, and with fine adjustments to center the 
source of light (crater of the positive carbon). 

2 First element of the condenser consisting of a meniscus and a plano- 
convex lens, to render the light beam parallel. 

3 Water-cell. 

4 Second clement of the condenser to converge the light-beam. 

5 Iris diaphragm to cut down the light-cone if desirable. 

6 Stage and substage condenser. 

7 Projection objective and fine focusing device. In the figure no ocular 
is used. 

This arrangement of the parts enables the user to employ a microscope with 
oculars or amplifiers, or the simple apparatus here shown, or photographic 
objectives. 



282 



HIGH POWER MICRO-PROJECTION 



[Cn. IX 



(2-3 mm. oil or water immersion) and for the 3 and 4 mm. dry 
objectives the condenser is usually very close up to the slide, so 
that the object is practically in the focus of the beam of light. 

For the 8, 10, 12, and 16 mm. objectives the substage condenser 
must be separated sufficiently from the specimen to light the whole 
field. 

It will be found in practice that one must be more precise in 
keeping the substage condenser at just the right level for projec- 
tion than for ordinary direct microscopic observation. Hence, it 
will be found that for a thin slide the condenser, even for high 
powers, may need to be separated slightly from the object, while if 
the slide on which the specimen is mounted is thick, the condenser 
may need to be as close to it as possible. 

408. Screen distance for high power projection. This 
should not be excessive, for even in the darkest room the image will 




FIG. 157. LEWIS WRIGHT'S PROJECTION MICROSCOPE. 

(From Wright's Optical Projection). 
C Condenser of three plano-convex lenses. 
-1 Alum eell for absorbing radiant heat. 

P Plano-concave lens of highly dispersive glass to aid in correcting the 
aberrations of the condenser and to render the light parallel. 

S C Substage condenser. For low powers but one lens is used. 

.V Stage. 

() Object and objective. 

A M Amplifier. 

F Fine focusing adjustment. 

R 2 Rack and pinion, coarse focusing adjustment. 

.ft, Coarse adjustment for the substage condenser. 



CH. IX] HIGH POWER MICRO-PROJECTION 283 

be too dim if the screen distance is over two or three meters (6 to 10 
feet). 

With objectives of 4, 6, 8, 10 mm. and lower powers, one can 
use a greater distance with satisfaction, but for the oil and water 
immersions, a distance of one to two meters (3 to 7 feet) gives the 
best results. This, of course, refers to minute details. If one 
simply wants size, the limit is much greater; but that is not 
scientific projection. 

409. Kind of screen for high power projection. The prin- 
ciple enunciated by Goring and Pritchard must be kept in mind. 
The whiter and smoother the screen, the more brilliant the image 
and the clearer the details. Nothing has been found better by 
the writers than smooth, white bristolboard. This is also very 
easily procured, and when it becomes dirty or discolored, it can be 
cheaply replaced. We have also found white cardboard in sheets 
of 71 x 112 cm. (28 x 44 in.) good. 

410. Specimens to project with high powers. These must 
have in a good degree the qualities of specimens giving clear images 
to the eye in direct, microscopic observation. That is, they should 
have definite outlines and contrasting colors; for example, well 
stained preparations of red and white blood corpuscles mounted in 
balsam and projected with the oil immersion objective. 

Preparations of bacteria, well stained and mounted in balsam, 
may be projected with the oil immersion. 

Thin histologic and embryologic sections, if well stained and 
mounted in balsam, answer well. The nuclei of cells show well, 
also the band of cilia in a ciliated epithelium, and the cells in 
mitotic division. Naturally, well prepared plant preparations 
have the advantage of very sharp outlines. 

411. High powers with the vertical microscope. Any prep- 
aration which can be projected well with high powers may be used 
on the vertical microscope ( 397). Of course, there is some loss 
of light in the double reflection required (fig. 147, 176), but if the 
screen is within two meters (6 ft.) distance and the observers few 
and close, results are fairly satisfactory. For example, if one has 



284 USE OF ALTERNATING CURRENT [Cn. IX 

water in which there are many large bacteria and infusoria, a most 
striking picture on the screen is made. For this projection a water 
immersion is excellent. An oil immersion may also be used and 
also a dry objective of 4 to 6 mm. 

For securing a large field, the objective and amplifier are better 
than an objective and ocular (355). 

USE OF ALTERNATING ELECTRIC CURRENT WITH THE PROJECTION 

MICROSCOPE 

412. It is unfortunate that it should ever be necessary to use 
alternating current in micro-projection ; but if that is all which can 
be obtained, much can be accomplished with it by skillful handling. 

(For a discussion of the difference between direct and alternating 
current and the relative amount of light yielded by the two, also 
for the possibility of getting direct from alternating current by 
means of a motor-generator set, or by a "current rectifier," see Ch. 

xin, 681-683, 751-752). 

413. Wiring the Arc Lamp. This is exactly as for the magic 
lantern, (fig. 3). And as with all arc lamp \vork there must always 
be present some form of regulating device like a rheostat or induc- 
tor (fig. 145, 197, 748). 

414. Arrangement of the carbons. For micro-projection the 
carbons should always be at right angles, and the light will then be 
almost wholly from the upper or horizontal carbon (fig. 191). As 
this is in the optic axis and looks directly toward the condenser it 
is the most satisfactory source of light available with this as with 
the direct current lam]) for micro-projection. This is because the 
image of the crater of one carbon is as large as can be received by 
the projection objective. 

It is especially necessary for micro-projection that the lamp have 
fine adjustments to keep the crater exactly centered (fig. 3, 14-6). 

415. Amount of current necessary. As the alternating 
current gives less than one-third as much available light as the 
direct current one cannot project with such high powers nor pro- 
duce so large screen images as with the direct current (fig. 302). 



CH. IX] MICRO-PROJECTION WITH HOUSE CURRENT 285 

For example, with direct current of 10 amperes one can accom- 
plish a great deal in micro-projection if the manipulation is skillful. 
To get equally brilliant results with alternating current would 
require 30 to 40 amperes of current. The heating is also excessive 
with the high amperages. (See Ch. XIII, 768). 

If alternating current must be used for projection with the micro- 
scope, one should not expect too much, but get as good results as 
possible by observing carefully the conditions giving good screen 
images, viz., apparatus in perfect order and alignment on one axis; 
a good screen and a dark room. 

It is not wise, according to our experience, to try to use more than 
25 amperes alternating current for micro-projection, and it is better 
as regards the specimens and apparatus, to be satisfied with the 
results which can be obtained with 15 to 20 amperes. An arc 
lamp with carbons at right angles is to be preferred. 

416. Centering the apparatus on one axis, separating the 
elements properly and the conduct of an exhibition are precisely 
as for the direct current light. The results, however, cannot be 
made as satisfactory, although, as stated above ( 412), by care and 
skill much can be accomplished. 

THE PROJECTION MICROSCOPE ON THE HOUSE ELECTRIC LIGHTING 

SYSTEM 

417. As with the magic lantern ( 127), the small electric 
current (4 to 6 amperes) available from the regular house lighting 
system gives very gratifying results. 

Small carbons (6-8 mm. diam.) are employed and either one 
of the small arc lamps especially designed for the purpose or an 
ordinary arc lamp with adapters or bushings can be used. 

Of course the direct current is much more effective, but even with 
the alternating current, which is now so common in lighting sys- 
tems, successful projection with the microscope can be done. 

The small carbons form a minute crater, and thus approximate 
closely to a point source of light, which is the ideally perfect source 
from the optical standpoint. From our experience this is a 



286 MICRO-PROJECTION WITH SUNLIGHT [Cn. IX 

better source of light for the microscope than the lime light, and 
now electric lighting is so common that one can use almost any 
room in a house or laboratory at night for a projection room. 

Of course one should not expect too much, but for small audiences 
50 to 100 and with a moderate sized screen 2-3 meters 
(6-10 ft.) astonishingly satisfactory micro-projection can be done. 

418. Hand-feed and automatic lamps for small currents. 

Most of the small current lamps are of the hand-feed type whatever 
the form of the electric current (a. c. or d. c.) but some automatic 
ones have been constructed (fig. 44, 205). Large arc lamps may, 
by special arrangement, be so adjusted that they give good results 
automatically from 5 to 25 amperes (e. g. the automatic lamp of 
A. T. Thompson and of the Bausch and Lomb Optical Co., fig. 
186, 187). 

As for the usual lantern arc lamps, only those for the direct 
current have hitherto been constructed of the automatic form. 

For a full discussion of the wiring and setting up of the apparatus 
see Ch. Ill and XIII, 128 and fig. 3, 40, 45. 

Do not forget that a rheostat or ballast of some kind must be 
used on every outfit where an arc lamp is employed ( 129, 748). 

Remember the precautions for turning on and off the current 
when using the house circuit ( 133). For a further use of these 
small currents in drawing, see Ch. X, 486. 

MICRO-PROJECTION WITH SUNLIGHT 

419. This was the first light used for micro-projection and 
remains the best. If it were only available at all times it would 
be universally employed. 

420. Arrangement of the parts of the apparatus. For the 

heliostat to keep the sunlight in a constant position one should 
consult Chapter VI. 

After getting parallel light from the sun in a constant position, 
then one should use the proper condenser (fig. 74). The remainder 
of the apparatus is precisely as for the projection so far discussed 
and all the requirements of centering and arranging at the proper 



CH. IX] MICRO-PROJECTION WITH LIME LIGHT 287 

distance from one another are as for the electric light described 
above. 

As the spot of light must remain in exactly the same place to be 
received by the small lenses of the projection objective, it is neces- 
sary to regulate the hand heliostats oftener than for the magic 
lantern. 

It may also be necessary to make slight corrections in the mirror 
of the clock-driven heliostat from time to time. The law is : The 
axial ray must correspond with the optic axis of the apparatus. 

421. Use of a water-cell. The radiant energy of the sun is 
so great that a water-cell to remove as much of it as possible except 
the luminous part ( 844) is as desirable as with the electric light. 
It is also desirable to have a specimen cooler (fig. 121). 

PROJECTION MICROSCOPE WITH THE LIME LIGHT 

422. The management of the lime light for the projection 
microscope is exactly as for the magic lantern (see 1 63 , 1 64) , only 
more attention will be necessary to keep the best possible light all 
the time. The image of the luminous spot should be focused on 
the hood of the objective as for the electric arc. While there is 
not so much danger from overheating as with the electric light or 
sunlight, it is desirable to use a large water-cell. The stage cooler is 
also an advantage. For the correct form of a condenser see 363. 

As the intrinsic brilliancy of the lime light is less than that of 
sunlight or the electric light one must not expect so much of it as 
of them. 

423. Other sources of light are insufficient to give good 
micro-projection except in a very limited degree, and for some 
special purposes. See under drawing, Ch. X, 463. 

HOME-MADE PROJECTION APPARATUS 

424. Projection table. For all kinds of projection the table 
should be of convenient height, so that the operator can stand dur- 
ing the exhibition. A height of 100 centimeters (40 inches) is 
suitable for most persons. The size of the top varies greatly with 



288 



HOME-MADE PROJECTION APPARATUS 



[CH. IX 



the work to be done. For the work of micro-projection, drawing, 
etc., contemplated in this and the following chapter a table of the 
following dimensions has served admirably: Height, 100 centi- 
meters (40 in.). Size of top 125 cm. (50 in.) long; 50 cm. (20 in.) 
wide. The legs are about 5 cm. (2 in.) square, and have large 
screw eyes in the lower ends for leveling. The table should be 




FIG. 158. HOME-MADE OPTICAL BENCH. 

t t t t The track of rods or tubes on the baseboard. 

Radiant The block carrying the arc lamp. 

as Asbestos paper between the track rods at the arc lamp end of the optical 
bench. 

Condenser The block carrying the condenser. 

Stage The block carrying the stage of the projection apparatus or the lan- 
tern-slide holder. 

Microscope The block carrying the projection microscope or the lantern 
slide or other projection objective. 

//// The railing flanges holding the sockets. 

base The baseboard. 

rigidly made so that there will be a minimum of vibration. If the 
table vibrates there is a disagreeable trembling of the screen image. 
(For pictures of such a table see fig. 133, 182). 

Carrying out the precautions against reflections from light sur- 
faces, the table is made dull black or brown. This is easily 
accomplished by using some dull black paint like "dead-black 
Japalac" or other dull black, or dull brown paint, thinned some- 
what with turpentine. 

The anilin black stain used for laboratory tables is also most 
excellent ( 424a). 

To the projection table should be fastened the rheostat, and the 
ammeter, if one is used ; also the lamp switch and the incandescent 
lamp (fig. 133). Then the table can be moved from one place to 



CH. IX] HOME-MADE PROJECTION APPARATUS 289 

another and be ready for projection by connecting the supply wires 
for the lamp to the line at any outlet box (fig. 3). 

425. Lathe bed or optical bench for projection apparatus. 

For the projection microscope, and for general experimental pur- 
poses there is no form of projection outfit so suitable and flexible 
as the lathe-bed type. It is easily and cheaply constructed. Any 
teacher with a little ingenuity and the aid of a tin-smith, black- 
smith, plumber, and carpenter or cabinet-maker, can construct all 
except the optical parts. The optical parts can be obtained of 
dealers or manufacturers of microscopes and projection apparatus. 
There is this further advantage in getting up a projection outfit, 
the person who does it will know enough to use it. He will not 

424a. Stain for laboratory tables. During the last few years an excellent 
method of dying wood with anilin black has been devised. This black is 
lustreless, and it is indestructible. It can be removed only by scraping off the 
wood to a point deeper than the stain has penetrated. 

It must be applied to unwaxed or unvarnished wood. If wax, paint or var- 
nish has been used on the tables, that must be first removed by the use of 
caustic potash or soda or by scraping or planing. Two solutions are needed : 

SOLUTION A 

Copper sulphate 125 grams 

Potassium chlorate or permanganate 125 grams 

Water 1000 cc. 

Boil these ingredients in an iron kettle until they are dissolved. Apply two 
coats of the hot solution. Let the first coat dry before applying the second. 

SOLUTION B 

Ani^in Oil 1 20 cc. 

Hydrochloric Acid 180 cc. 

Water 1000 cc. 

Mix these in a glass vessel putting in the water first. Apply two coats with- 
out heating, but allow the first coat to dry before adding the second. 

When the second coat is dry, sandpaper the wood and dust off the excess 
chemicals. Then wash the wood well with water. When dry sandpaper the 
surface and then rub thoroughly with a mixture of equal parts turpentine and 
linseed oil. The wood may appear a dirty green at first but it will soon become 
ebony black. If the excess chemicals are not removed the table will crock. An 
occasional rubbing with linseed oil and turpentine or with turpentine alone will 
clean the surface. This is sometimes called the Danish method, Denmark black 
or finish. See Jour. Ap. Micr., Vol. I, p. 145; Bot. Zeit., Vol. 54, p. 326, Bot. 
Gazette, Vol. 24, p. 66, Dr. P. A. Fish, Jour. Ap. Micr., Vol. VI., pp. 211-212. 
The Anatomical Record, Vol. V. 191 1, pp. 145-146. (Quoted from The Micro- 
scope, by Gage, nth cd. 1911, pp. 282-283). 



2QO HOME-MADE PROJECTION APPARATUS [Cn. IX 

expect the apparatus to do the work of a machine, and also to 
supply all of the intelligence to enable it to do so. 

426. Baseboard and track. For the lathe-bed carrying all 
the apparatus (fig. 121, 159) a flat board about 2 cm. (J/gin.) thick 
is used for the base. The width and length can be made to suit 
the apparatus designed. The dimensions for that shown in fig. 
i58-i59are: Length 125 cm. (4 ft.); width 22.5 cm. (8^ in.). 

The track which serves as a guide to the blocks bearing the differ- 
ent pieces of apparatus (fig. 121) is best made of two brass tubes or 
rods 12 mm. (^ in.) in diameter and the full length of the base- 
board ( 42 6a). 

427. Fixing the track to the baseboard. For this, holes 
should be bored through the tubes or rods, being careful to have 
the holes parallel so that there will be no torsion or twist when the 
tubes are fastened to the board. If rods are used the screw holes 
must be countersunk. If tubes are used then the upper wall 
should have a larger hole than the lower and a slender screw driver 
used, (fig. 159 ts), then the screw head goes through the upper wall 
and presses against the lower side only. 

One tube or rod is fixed firmly to the base, thus : With a straight 
edge like a T-square make a straight line on the baseboard where 
the track is to be laid and then fasten the one track accurately along 
this line so that it will be perfectly straight. 

Now for the other track lay it as follows : Use apparatus blocks 
( 428) near the ends of the baseboard and put the loose rod in 
place. Press the block down firmly so that the loose track will be 
forced into the groove. Put screws in the end holes, but do not 
screw them down firmly. If there arc intermediate holes as in 
fig. 158-159 move a block near the hole, press it down firmly and 
then put in a screw, but do not screw it in firmly. 

426a. For the rods, one ean procure the thin, polished or nickeled brass 
tubing used for railing, or the thick brass tubes used instead of iron tubing. The 
measurement given means the total diameter. Of course one can use any 
desired diameter by varying the' size of the V-shaped notches in the apparatus 
blocks (fig. 158 A) or the position of the cleats (fig. 159). If brass tubing is 
employed for the track, the size known to the plumber is that of the bore, not 
the outside diameter. Tubing \vith x ' 4 th or 9'ijtli inch bore answers well. The 
outside diameters will be 10 and 13.5 mm. (13/32 and 17/32 in.) respectively. 



CH. IX] HOME-MADE PROJECTION APPARATUS 291 

This will make a track along which the blocks will move freely. 
If both tracks were firmly fixed the blocks would have to be con- 
structed with extreme precision or the blocks would bind. They 
would also bind if the tracks were not perfectly parallel at all 
points. The loose track gives slightly and thus compensates for 
any little irregularity of the track or apparatus block. 

428. Apparatus blocks. These are shown in figures 158, 159. 
They must be sufficiently heavy so that the various pieces of appara- 
tus they carry will be steady ; and finally the sockets for receiving 
the stems of the apparatus must be on the blocks in a position so 
that the parts like the stage and the microscope can be brought 
sufficiently close together. 

Size and weight of the different blocks for the apparatus figured 
(fig. 158, 159): 

1. Arc lamp block. 12^ x 12^ cm. (5 x 5 in.) ; weight i kilo. 
(albs.), (fig- 158). 

2. Condenser block. 12^ x 10 cm. (5 x 4 in.) ; weight 2 kilos. 
( 4 # Ibs.), (fig. 158). 

3. Stage block, 12^ x 6 cm. (5 x 2 in.) ; weight, i kilo. (2 Ibs.), 

(fig- 158). 

4. Microscope block, 12^2x10 cm. (5x4 in.); weight, 2-3 
kilos. (4-6 Ibs.), (fig. 158). 

5. Block for lantern-slide carrier, 12^2x6 cm. (5 x 2 in.); 
weight Y 2 kilo, (i lb.), (fig- 158). 

6. Block for lantern objective, or a photographic objective, 
(fig. 158), i2>^ x 10 cm. (5x4 in.); weight, 2 kilos. (4^ Ibs). 

7. Block for horizontal microscope, 17 x 12^ cm. (7x5 in.), 
weight 2^/2 kilos. (5^ Ibs.), (fig. 145). 

429. Construction of the apparatus blocks. If one has the 

facilities of a machine shop and foundry at his disposal these 
apparatus blocks may be made of cast iron, smoothed and grooved 
on a planer. In like manner the lathe bed with V's can be made 
(fig. 134). Lacking these facilities one can prepare blocks of wood 
which will answer almost perfectly as follows: Select some fine 
grained board 2 cm. to 2.5 cm. thick ( 7 /$ to i in.), and cut it into 



HOME-MADE PROJECTION APPARATUS 



[Cn. IX 



blocks of the required size for the special purpose. The blocks can 
be made as heavy as desired by adding sheets of lead (fig. I58A). 

For the guides to follow the track, one can make V-shaped 
grooves, or more easily, strips of the proper thickness can be screwed 
to the block (fig. 159). One of the strips should be screwed tightly 
to the block, and the other should have screw holes through the 
strip considerably larger than the screws, then it will be possible to 
make slight changes in position to get an exact fit. When in the 
exact place desired the screws can be set firmly. As the large holes 
in the strips will be larger than the heads of the screws, metal 
washers should be employed (fig. 159). 

430. Sockets for the stems of the apparatus. There is first 
screwed to the top of the block, a railing flange, and into this is 
screwed a short tube of the size to receive the stem or post of the 




FIG. I58A. SHIELD FOR A PROJECTION ORJECTIVE. 

This shows the method of supporting an objective in a shield. The shield is 
supported by a bolt with a fan shaped end (p). 

The bolt or stem enters the socket and is held in place by a screw (s). 

U Apparatus block composed of U-ad sheets above and a block of wood 
below. The block of wood has V-shaped grooves for sliding along the track. 

b Baseboard with track, end view. 



CH. IX] 



COMBINED PROJECTION 



293 



apparatus. This tube has a set screw in the side to hold the post 
at any desired level (fig. is8F). In order to be able to perfect the 
centering of the apparatus, the screw holes in the flanges are made 
larger than the screws so that by loosening the screws the flange 
can be shifted slightly from side to side. If necessary one can use 
washers to increase the size of the screw heads, so that the holes 
in the flange can be quite large. 

431. Wooden shields for holding objectives, etc. For hold- 
ing projection objectives of low power, shields of thin board (i to 




FIGURE 158?. SECTIONAL VIEW OF A RAILING FLANGE AXD SOCKET. 

/ The flange in section. The screw holes are made large for centering. 
5 The set screw to hold the post in place. 

p Post extending down into the socket. It is held at any desired height 
by the set screw (s). 

sc Socket for receiving the post or stem of any piece of apparatus. 

\ l /2 cm., l /2 in. thick) can be used, and a post or stem of iron made 
from a bolt by hammering out the end in the form of a fan (fig. 
i58A). To aid in centering, the screw holes in this post should 
also be larger than the screws. 



MICROSCOPE AND LANTERN-SLIDE PROJECTION COMBINED 

432. With an outfit of the lathe-bed type (fig. 138), it is very 
simple to change from lantern-slide to micro-projection and the 
reverse. All that is necessary is to put the lantern-slide carrier 
next the condenser, and the lantern-slide projection objective on its 
block in position. The stage and the microscope must be set off 
the track on the table. The only difficulty is that the second ele- 
ment of the condenser for the micro-projection is of too short a 
focus for most lantern-slide projection. This can be overcome in 



294 



COMBINED PROJECTION 



[CH. IX 



table 




8 b 2 



(yip 



b3 





CD 






;< ii 


,'9; 


b4 



c w (t) 



Base Board 



Base 




Board 




t t 








e 







e 

















b1 














a 







e 


























e 










b2 

















e 























e 










b 3 

















e 


















e 1 


e 














b4 













e 










t t 




Base 




Board 



Fu;. 159. HOME-MADE OPTICAL BENCH KOR ALL PURPOSES. 

Base Board This is drawn at the right as if transparent to show the track 
and under side of the carrying 1 flocks, and the various ways in which the guid- 
ing cleats can be applied. 

t t t t The tubular tracks on which the carrying blocks ride. 



CH. IX] 



COMBINED PROJECTION 



295 



b i Block with four guiding cleats of wood. The screw holes in the inside 
cleat at the left and the outside one at the right are made large, and washers 
are used on the screws. This is to make accurate centering possible. 

b 2 Block showing two guiding cleats between the tracks. Only one 
cleat has large screw holes for centering. 

b j The third carrying block with the guide cleats on the outside of the 
track rods. Only one has large screw holes for centering. 

b 4 The fourth carrying block with guide cleats at only one end, and with 
centering holes in one cleat. 

At the left Sectional views of the carrying blocks. 

In b I is shown how to make a table for carrying apparatus along the optical 
bench, and at / s, the method of screwing the track tubes to the baseboard. 

In b 4 is shown how to attach a shield with an opening for lantern slides (0). 

two ways: (i) The arc lamp can be put closer to the condenser, 
thus making the beam between the elements diverging instead of 
parallel (fig. i), or (2) a condenser lens of longer focus can be used 
for the lantern-slide projection. In much of the modern projection 
apparatus the condenser lenses are easily changed (see fig. 166). 





FIG. 1 60. UNIVERSAL LEVEL. 
(Cut loaned by the L. S. Starred Co.). 

A level like this which serves for vertical and horizontal leveling is very 
convenient and essential for projection work. 



The second method of combined projection is to have two com- 
plete lanterns side by side, one for micro-projection and one for 
lantern-slide work. In this case there should be a double- pole, 
double-throw switch ; then one can turn either lantern off or on at 
will (fig. 162, 164). 

Finally, in much of the modern apparatus special provision is 
made for combined projection (see fig. 164-176). 



296 PROJECTION MICROSCOPES ON THE MARKET [Ctf. IX 




FIG. 161. THUMB SCREWS AND THUMB NUTS. 

(From the Catalogue of the Hartford Screw Company). 

Thumb screws and thumb nuts are necessary if one is to construct home- 
made apparatus. 



PROJECTION MICROSCOPES OBTAINABLE IN THE OPEX MARKET 

433. The projection microscope so far considered in this 
chapter was designed to give the range needed for modern micro- 
projection in a biologic or other laboratory, that is, for use with 
specimens slightly smaller than lantern slides (50 to 65 mm. in 
diameter) to those of i mm. or less (fig. 121, 147). 



CH. IX] 



COMBINED PROJECTION 



297 



Screen Image 




Will W2 

FIG. 162. COMBINED LANTERN-SLIDE AND MICRO-PROJECTION WITH 
Two COMPLETE OUTFITS SIDE BY SIDE. 

Wi W2 The supply wires from the outlet box (fig. 3). 
D S Double-pole, double-throw knife switch. 
/ Binding post of supply wire W2. 
2 Binding post for supply wire W i. 

j Binding post for the wire (W j) from the switch to the rheostat at the 
right. 

4 Binding post for the wire (W 5) to the lower carbon of the arc lamp at the 
right. 

5 Binding post on the switch for the wire (W 6) to the rheostat at the left. 

6 Binding post on the switch for the wire (W 5) to the lower carbon of the 
arc lamp at the left. 

II i, H 2 Hinges for the switch blades. 

j i, j 2, j 3, j 4 Jaws for receiving the switch blades when the switch is 
closed. 

SH Switch handle for opening and closing the switch. The switch is 
closed at the right. On the left the handle, bar and switch blades are shown 
with dotted lines. 



298 PROJECTION MICROSCOPES ON THE MARKET [Cn. IX 

W 8 The wire to the left lamp, lower carbon. 

W 6, W 7 Wire including the rheostat, passing to the upper carbon of the 
left arc lamp. 

L Rheostat Rheostat for the left lamp. 

r 3, r 4 The binding posts of the left rheostat. 

L Lamp The left arc lamp. 

F Feeding mechanism for the carbons. 

d Clamp for fixing the arc lamp in any vertical position on its standard. 

5 s Set screws for the carbons. 

// C Horizontal or upper carbon. 

V C Vertical or lower carbon. 

R Rheostat Rheostat for the right arc lamp. 

r i, r 2 Binding posts for the rheostat. 

W 3 W 4 Wire from the switch through the right rheostat to the upper 
carbon of the right arc lamp. 

W 5 Wire to the lower carbon of the right lamp. 

R Lamp The right lamp. It is exactly like the left one. 

L Lamp, R Lamp The arc lamps for the two projectors. 

Condensers The triple-lens condensers with water-ceils for the two pro- 
jectors. 

Axis, Axis, Axis, Axis Principal optic axis in the two projectors. 

P Objective The projection objective at the left. 

Microscope The projection microscope at the right. 

Screen Image, Screen Image The images formed on the screen by the two> 
instruments. 

NOTE. In using these projectors it is only necessary to turn the switch 
handle over to the one desired and that lamp can be lighted. One can turn 
from one to the other at will. 

A more economical arrangement would be to have a single rheostat inserted 
along either Wi or W2 before reaching the knife switch, then the single rheostat 
would serve for both lanterns. 

With the two rheostats, as here shown, both lanterns could be run at the 
same time if there were two switch handles and double blades hinged at the 
center (II i, H 2). 

The projection microscopes in the open market rarely possess 
anything like this range. Very few will project an object as great 
as 25 mm. in diameter. 

It seems to the writers of this book that the makers have unduly 
limited the range of their apparatus by a too rigid insistence on the 
use of substagc condensers and projection oculars, and also by the 
effort to make combined apparatus. Combination always means 
compromise and more or less loss of individual efficiency. 

It is certain, too, that most of them have not fully appreciated 
the necessity for dull black surfaces. The bright finish is probably 
to please the eye when the apparatus is not in operation. It 
certainly is not good for the eyes when the apparatus is in opera- 
tion. 



CH. IX] PROJECTION MICROSCOPES ON THE MARKET 299 

However, many opticians are coming to finish their apparatus 
in black, and all of them are ready to make modifications in their 
instruments which they are convinced will make them more effec- 
tive and convenient for those who are to use them. But as many 
men have many minds it is not possible for the manufacturers to 
please every one in all particulars, hence the apparatus in the open 
market must represent a kind of average. While the authors 
realize the limitations mentioned above, it is a pleasure to be able 
to assert without reserve that the quality and design of the appa- 
ratus obtainable at the present time are excellent. 

434. As the projection microscopes most common in America 
are of German, English and home manufacture some examples are 
illustrated below. 




FIG. 163. LEITZ PROJECTION MICROSCOPE. 

(From Leitz Catalogue}. 

1 Arc lamp. 

2 Condenser next the arc lamp. 

3 Water-cell. 

4 The lantern-slide holder. 

5 Iris diaphragm. 

6 Biconcave, illuminating lens to give the light the right angle before it 
enters the substage condenser. 

7 Stage and substage condensers, on a revolver for use with different powers. 

8 Projection objectives on a revolving nose-piece. 
Q Projection oculars on a revolver. 

The enclosing curtain is turned over the top to uncover the parts. (Sec fig. 
96 for the entire apparatus in its latest form, 1914). 



300 PROJECTION MICROSCOPES ON THE MARKET fCn. IX 




FIG. 164. PROJECTION MICROSCOPE, MAGIC LANTERN AND MEDIOSCOPE 

WITH SINGLE RADIANT. 
(Cut loaned by Williams, Broivii & Earle). 

The arc lamp and condenser move laterally so that each instrument can he 
illuminated at will. 

A The medioscope is an achromatic combination of large aperture for 
objects of large size, but smaller than lantern slides. 

B Projection microscope with large projection ocular. It has a water-cell 
(D) in the path of the light. The arc lamp and condenser (N O P) are in place 
for micro-projection. 

C Projection objective for lantern slides. 




FIG. 1 6s. 



NE\V REFLECTING LANTERN WITH THE PROJECTION 
MICROSCOPE. 



CH. IX] 



TROUBLES WITH MICRO-PROJECTION 



301 



removed. For projection the mirror must be in place to reflect the light along 
the axis as for lantern slides. The change to the projection of opaque objects 
is almost instantaneous, but for lantern-slide projection the projection micro- 
scope must be removed and the lantern-slide objective put in place, but^the 
apparatus is so constructed that this is easily accomplished. 




FIG. 166. IMPROVED, COLLEGE, BENCH LANTERN ARRANGED FOR 

M ICRO-PROJECTION. 
(Cut loaned by the Mclntosh Stereopticon Co.). 

The optical bench consists of a long baseboard with the two guide rods 
supported by three brackets. 

As each part is independent it can be changed in position or entirely removed 
and other apparatus put in its place, thus giving great flexibility. 



TROUBLES WITH THE PROJECTION MICROSCOPE 

435. The source of troubles with the projection microscope 
are mainly the same as with the magic lantern. These have been 
fully discussed at the end of Chapter I ( 62-98). See i28a for 
the blowing of fuses with the arc lamp on the house system. 

The special troubles with the projection microscope are almost 
wholly due to the smallness of the lenses necessary for micro-pro- 
jection; and as the foci of these lenses are relatively short, slight 
changes in the position of one of the elements of the apparatus, and 
slight deviations from the true axis produce correspondingly great 
effects. It is necessary to be more exact in micro-projection, but 
the great fundamental principles are exactly as for the magic 
lantern. 

436. Insufficient illumination on the screen. Besides those 
given in Chapter I the following may be causes : 
i. Too large a screen image may be attempted. 



302 



TROUBLES WITH MICRO-PROJECTION 

1G 



[CH. IX 




FIG. 167. UNIVERSAL PROJECTOSCOPE SHOWING THE ARRANGEMENT 
FOR HORIZONTAL TRANSPARENCIES AND FOR MICROSCOPIC PROJECTION. 

(Cut loaned by the C. H. Stoelting Co.}. 
Commencing at the left : 

1 Feeding screws for the carbons. 

2 Fine adjustment for moving the arc lamp back and forth along the axis. 
3-4 Fine adjustment screws for moving the arc vertically and laterally to 

keep the crater in the axis. 
L H Lamp and lamp-house. 
C First element of the two-lens condenser. 
F-F vSupports. 
T Water-cell. 

M Mirror above the objective to reflect the light to the vertical screen. 
M, M, Mirror in position to reflect the horizontal beam directly upward. 
C, C 3 Second element of the two-lens condenser. 
R Projection microscope. 

B Optical bench on which slide the different pieces of apparatus. 
B l B, Supports of the optical bench. (See also fig. 16, 102). 

2. The object may not be in the best position in the light cone 



3. The substagc condenser, when that is used, may be a little 
too near or too far from the specimen. Slight changes in 



CH. IX] 



TROUBLES WITH MICRO-PROJECTION 



303 




FIG. 168. THOMPSON'S PROJECTION MICROSCOPE. 
(Cut loaned by the A. T. Thompson Co.). 

The projection microscope with the substage condenser system is attached 
to the reflectoscope (rig. 97) in the position where vertical opaque objects are 
placed ; this allows the direct beam of light to be utilized in micro-projection. 

The stage and the objective holder are independent, and no ocular is used. 
This permits the projection of large objects with low powers or smaller objects 
with high powers. From the short tube employed, the field is not restricted. 

its position often work wonders. The substage condenser 
may be too near to the large condenser or too far from it 
so that the light cone does not reach it in its most favorable 
position. 

4. The room may not be dark enough or external light may fall 

directly on the screen from some window or open door. 

5. Never forget the carbons. A slight mal-position or decen- 

tering of the crater may cause all the trouble. 



304 



PROJECTION AIICROSCOPES ON THE MARKET [Cn. IX 




(Balance of descriptive matter on next page} 



CH. IX] PROJECTION MICROSCOPES ON THE MARKET 305 

Commencing at the left : 

Large, well ventilated, light-tight lamp-house. As shown in fig. 104, 105, 
the lamp-house with the lamp and first element of the condenser can be inclined 
to direct the light downward upon an opaque object. 

Following the lamp-house is a dark box for opaque projection. The large 
projection objective with mirror is above and the table for the opaque objects 
below. Within the dark box is a mirror so inclined that it reflects part of the 
scattered light back upon the object (see also fig. 105). Opaque objects up to 
20 cm. (8 in.) square can be projected. 

Following the large objective for opaque projection is an objective for lantern- 
slide or other projection with the object in a horizontal position. Following 
this is the polarizing apparatus of glass plates (see 880). The second element 
of the condenser serves for lantern-slide and for low power micro-projection, 
but for high powers this is turned out as here shown and a small double convex 
lens in the dark chamber near the first element of the condenser is swung into 
position and serves to project an image of the crater at the plane of the dia- 
phragm of the substage condenser (fig. 170). 

Just beyond the bellows are shown the projection microscope and the 
lantern-slide objective. These are so hinged parallel to the axis that the 
microscope can be turned laterally and thus bring the lantern-slide objective in 
position. In the picture the lantern-slide objective is turned aside and the 
projection microscope is in position. 

The substage condensers for different objectives are shown on a revolving 
carrier, as are also the micro-projection objectives and the projection ocular 
and amplifier. 




FIG. 170. DIAGRAM OF THE ILLUMINATING SYSTEM FOR HIGH POWER 

PROJECTION. 
(Cut loaned by the Bausch & Lomb Optical Co.}. 

This is a modification of the Kohler system ( 401-403), and consists of the 
first element of the triple condenser (meniscus and convex lens) to render the 
beam parallel. The small, convex lens near the condenser serves to project 
an image of the crater upon the plane of the diaphragm of the substage con- 
denser. This is designed to fill the aperture of the substage condenser and, 
hence, of the high power objectives. 

L The radiant. 

C The meniscus and convex lens of the condenser and the small special 
convex lens for micro-projection. 

L' Inverted image of the radiant. 

E Substage condenser. 

5 The specimen. 

C' Image of the small condensing lens in the plane of the specimen (5). 



306 



PROJECTION MICROSCOPES ON THE MARKET [Cn. IX 




CH. IX] PROJECTION MICROSCOPES ON THE MARKET 307 

FIG. 171. NEW STYLE CONVERTIBLE BALOPTICON FOR MICROSCOPE, LAX- 
TERN-SLIDE AND OPAQUE PROJECTION, AND FOR THE PROJECTION OF 
LARGE TRANSPARENCIES IN A HORIZONTAL POSITION. 

(Cut loaned by the Bausch & Lomb Optical Co.). 

As shown in the picture, this instrument is designed for projecting all kinds 
of objects either in a vertical or in a horizontal position. For the large trans- 
parencies the object is placed on the broad plate beneath the objective. 
Immediately under the object is the condenser lens of 20 cm. (8 in.) diameter, 
thus making it possible to project X-Ray plates, brain sections, etc., 20 cm. 
(8 in.) in diameter. A mirror in the dark chamber directs the horizontal beam 
from the first element of the condenser vertically as in all projection of this 
kind. 

For the large transparencies the projection objective is in a vertical position 
with mirror to reflect the light to a vertical screen and to overcome the left to 
right inversion. 




FIG. 172. UNIVERSAL BALOPTICON FOR OPAQUE OBJECTS, MICROSCOPIC 

OBJECTS AND FOR LANTERN SLIDES OR OTHER TRANSPARENT OBJECTS 

IN A VERTICAL OR A HORIZONTAL POSITION. 

(Cut loaned by the Bausch & Lomb Optical Co.). 

For the opaque projection and lantern slides in a vertical position see fig. 106. 

For lantern slides or other transparent objects in a horizontal position the 
arrangement for vertical slides is pushed back, and this brings the condenser 
lens and plate for supporting horizontal objects over the opening. The same 
mirror is used for directing the beam of light upward as for the vertical slides. 



308 TROUBLES WITH MICRO-PROJECTION [Cn. IX 

For micro-projection the microscope is placed in line with the objective for 
opaque objects, the objective serving as a condenser. The light passes directly 
from the radiant through the first element of the condenser and the objective 
for opaque objects to the microscope. The microscope is so hinged that it can 
be turned aside and the other forms of projection quickly brought into use. 

6. There may be mist on some of the glass surfaces as the water- 
cell, or some glass surface like the objective front may be 
dirty. 

437. Unequal illumination of the screen. This is often due 
to the lack of centering of some element. 

1. It is usually the crater of the upper carbon that gets out of 

the axis. It is easily corrected by means of the fine adjust- 
ments of the lamp (fig. 3). 

2. There may be some less transparent part of the object over 

part of the field. One can easily determine this by moving 
the specimen slightly. 

3. Part of the mask ( 384, fig. 143, 148) may be in the field. 




FIG. 1 73. BAUSCH & LOME'S SIMPLEST FORM OF PROJECTION MICROSCOPE. 
(Cut loaned by the Batisch & Lomb Optical Co.). 

This is designed for low power projection and consists of an objective holder, 
rack and pinion focusing adjustment, stage and substage condenser for low 
powers. The whole is put in place of the projection objective for lantern slides. 
This simple outfit added to the magic lantern enables one to do very successful 
micro-projection. 

438. Hazy images may be due to direct light on the screen 
from some window, etc. Keep especially in mind also that internal 
reflections in the objective, the microscope tube or the amplifier 
tube will cause hazy images (370, 371), also dirt or balsam on the 
front lens of the objective. 



CH. IX] 



TROUBLES WITH'MICRO-PROJECTION 



309 



439. Dark spots on the screen. 

i. They may be caused by air bubbles in the water-cell or in 
the stage cooler. 




FIG. 174. SIMPLE ADDITION TO THE MAGIC LANTERN FOR 
MICRO-PROJECTION. 

(Cut loaned by the Spencer Lens Co.). 

This consists of a jointed frame by which the objective holder and focusing 
device can be brought down in position when the lantern-slide objective is 
turned aside. No microscope tube is used. This makes a very efficient and 
convenient addition to a magic lantern at moderate cost, and with it a great 
deal of projection can be successfully accomplished. 

For lantern-slide projection the microscope is turned to the top of the lamp 
enclosure and the lantern-slide objective is turned on its hinge back into posi- 
tion in the optic axis. 



2 . They may be caused by dark spots or bubbles in the slide or 

specimen. 

3 . Dark spots on the condenser, amplifier or ocular may cause 

them. 

440. General conditions for good micro-projection. With 
good specimens, clean glass surfaces, and all the elements on one 
axis, there should be no trouble in getting a good screen image on a 
suitable screen and in a well darkened room. 

It would be of very great advantage for any man who aspires to 
use the projection microscope effectively, if he could see the room, 
apparatus, and exact method of work of some one who had mas- 
tered the art. Good projection will not do itself. 



PROJECTION MICROSCOPES ON THE MARKET [Cn. IX 




FIG. 175. MODEL 4-5 DELINEASCOPE WITH THE MICROSCOPE IN A VERTICAL 
POSITION* FOR HORIZONTAL OBJECTS. 
(Cut loaned by the Spencer Lens Co.). 

This figure is to show the course of the rays for lantern slides in a vertical 
position and for microscopic objects in a horizontal position. 

A mirror M reflects the light vertically through the horizontal specimen, and 
by means of a prism (PR) in the tube of the microscope the vertical light is 
made to extend out horizontally to the screen. 

A joint in the microscope frame makes it possible to turn the microscope 
down in front of the instrument after turning the lantern-slide objective aside 
on its hinge. Then vertical objects can be projected in the usual manner, or 
by using the prism (PR) the image can be reflected down upon a horizontal 
drawing surface. 



CH. IX] PROJECTION MICROSCOPES ON THE MARKET 




FIG. 176. MODEL 8 DELINEASCOPE SHOWING THE POSITION OF THE 
MICROSCOPIC ATTACHMENT FOR VERTICAL AND FOR HORIZONTAL 

OBJECTS. 

(Cut loaned by the Spencer Lens Co.). 

This projection microscopic attachment is designed to use with or without 
oculars or amplifiers, and for microscopic objectives of all foci from 125 
mm. to the highest available. The substage condenser consists of several 
lenses which are easily turned in place or out of position. By making a suitable 
combination any object and any objective can be used. To enable the opera- 
tor to get the object in the right position in the cone of light there is a rack and 
pinion movement moving microscope and stage toward or from the condenser. 
Th ; s is done by the lower milled head shown. The upper milled head is for 
the usual coarse adjustment and a micrometer screw is present for the fine 
adjustment (see also fig. 177). 



312 



PROJECTION MICROSCOPES ON THE MARKET [Cn. IX 




FIG. 177. DIAGRAM SHOWING THE COURSE OF THE RAYS FOR LANTERN 
SLIDES AND FOR MICROSCOPIC OBJECTS IN A. VERTICAL AND IN A 
HORIZONTAL POSITION WITH MODEL 8 DELINEASCOPE. 

(Cut loaned by the Spencer Lens Co.). 

T Table for opaque objects. 

W Wheel by which the table is raised and lowered. 

D Diaphragm which may be used above the table. 

B Bulb which always illuminates the interior of the machine. 

C Condensing lenses in front of the arc. 

Large objective for opaque work. 

1 Smaller objectives for vertical attachment. 

M Mirror for throwing light downward to the lantern slide. 

M l Mirror for throwing a perpendicular beam out through the lantern- 
slide compartment. 

M., Mirror used in connection with the projection of the vertical side of an 
object. 

M., Mirror which assumes a position of 45 when the microscope is used 
perpendicularly. 

P Prism which is used in the prism chamber when the microscope is used 
perpendicularly or for drawing on a horizontal surface when the microscope 
is horizontal. 

S .Shelf upon which the lantern slide is placed previous to throwing it up 
into the optical axis by the handle. 

II Handle of the lever for raising lantern slides into position. 



CH. IX] 



DO AND DO NOT IN MICRO-PROJECTION 



313 



441. Summary of Chapter IX: 



Do 

1. Use actual objects in lec- 
tures and discussions as well as 
diagrams (352). 

2. Employ a projection micro- 
scope with equipment for speci- 
mens ranging from 60 mm. to 
less than i mm. in diameter 

(354). 

3 . In demonstrating with the 
projection microscope use first a 
low power and show the rela- 
tions of parts, then use higher 
powers to show details. 

4. Use objectives without 
oculars from 125 mm. to 4 mm. 
focus (355)- 

5 . Oculars or amplifiers can be 
used with all the objectives on 
the microscope (fig. 138), but 
preferably with those not higher 
than 8 mm. focus. 

6. Use a screen distance from 
5 to 10 meters (16 to 33 feet). 

7. It is better to use a micro- 
scope in the usual manner if 
very high powers, like the oil 
immersion, are to be used (355). 

8. If possible use a triple- 
lens condenser ( 363). 



Do NOT 

1 . Do not stop with diagrams 
where actual specimens can be 
shown. Diagrams alone are 
liable to give false impressions. 

2. Do not use projection 
apparatus with a narrow range 
of field or of powers. 



3. Do not show minute de- 
tails without first showing the 
object as a whole, so that rela- 
tions can be clearly recognized. 

4. Do not use oculars for 
projecting for large, class demon- 
strations. Oculars restrict the 
field too much. 

5. Do not use oculars or 
amplifiers unless for special 
reasons. 



6. Do not have the screen 
distance too great. 

7 . Do not try to make out the 
finest details by projection, but 
use a microscope in the ordinary 
way. 

8. Do not use a poor con- 
denser for micro-projection, the 
triple form, meniscus next the 
radiant, is best. 



DO AND DO NOT IN MICRO-PROJECTION [Cn. IX 



9. Make the room dark and 
use a perfectly white image- 
screen for micro-projection 
( 3 6 o)- 



9. Do not try to project with 
the microscope in a room that 
cannot be properly darkened or 
with a dirty screen. 



i o . Use only the direct current 
arc light for micro-projection, 
unless compelled to use alter- 
nating current ( 412). 

11. Use an ammeter and a 
variable rheostat or other bal- 
ancing device and be very 
careful about the wiring (fig. 2-3 
188). 

12. The arc lamp must have 
fine adjustment screws to enable 
one to keep the arc centered on 
the objective front ( 362). 

13. Always use a water-cell 
in micro-projection with the arc 
lamp radiant ( 364). 



14. Use a mechanical 
for serial sections ( 366). 



stage 



15. Blacken the objective 
mounts, and all metal parts of 
the projection apparatus to 
avoid glare; make sure there 
arc no shiny surfaces within the 
projection apparatus ( 570- 



10. Do not use alternating 
current if it is possible to obtain 
direct current. 



n. Do not neglect the am- 
meter and the variable rheostat 
when installing a projection 
microscope. 



12. Do not try to use an arc 
lamp without fine adjustments, 
otherwise the crater cannot be 
kept centered. 

13. Do not project with the 
arc lamp without using a water- 
cell to absorb the radiant heat. 

14. Do not try to show 
selected sections of a series 
without the help of a mechani- 
cal stage. 

15. Do not leave the objec- 
tive mounts with brilliant re- 
flecting surfaces to dazzle the 
eyes of the operator, and do not 
leave shiny surfaces within the 
apparatus to give cross lights 
and make the image dim. 



CH. IX] DO AND DO NOT IN MICRO-PROJECTION 



315 



1 6. Use a hood on the objec- 
tive to aid in centering the light 
and in placing the objective the 
right distance from the conden- 
ser ( 372); a light shield 
beyond the objective to stop 
stray light is also an advantage 
( 373)- 



1 6. Do not forget the advan- 
tages of an objective hood for 
centering the light and prevent- 
ing glare; and do not omit the 
light shield to cut off stray light. 



17. It is of the utmost im- 
portance that every part be 
accurately centered for micro- 
projection ( 375), and that the 
parts should be separated from 
one another the right distance 
( 376, 382). 



17. Do not fail to have all 
parts accurately centered, and 
the correct distance apart. 



1 8. Remember that it is a 
pure waste to use too great an 
amperage ( 378). 



1 8. Do not use a greater cur- 
rent than necessary. 



19. As the same object is to 
be shown entire and with magni- 
fied details and different objects 
require different magnifications, 
it is convenient to have two, 
three or four objectives of 
different powers in a revolving 
nose-piece ( 379). 

20. For exhibition purposes 
it is a great advantage to use 
carbons whose ends have been 
shaped by previous burning in 
the lamp ( 380). 



19. Do not show all objects 
with the same objective, but 
have two or three on a revolving 
nose-piece so that different 
powers can be used with the 
minimum of trouble. 



20. Do not forget to shape 
the ends of the carbons by burn- 
ing them awhile in the arc lamp 
before anv formal exhibition. 



DO AND DO NOT IN MICRO-PROJECTION 



[CH. IX 



21. Be sure that the carbons 
are in the correct mutual posi- 
tion to give a good light. A 
screen image of the burning 
carbons often is of real help 
(381). 



21. Do not omit the correct 
setting of the carbons. A good 
light cannot be produced with 
the carbons in the wrong mutual 
relation. 



2 2 . Mask the preparations for 
exhibition ( 384). 



23. Remember the advan- 
tages of a large field for seeing 
the relation of parts (387). 



2 2 . Do not exhibit specimens 
which are not properly masked. 
It is necessary to be able to 
work with certainty and rapid- 
ity in an exhibition. 

23. Do not forget the impor- 
tance of a large field so that the 
relations of parts can be seen. 



24. Remember that one can 
do good projection work with 
an ordinary microscope ( 393). 

25. For objects which must 
remain in a horizontal position, 
a vertical microscope must be 
used; this involves the use of 
two mirrors or of a mirror and a 
prism to reflect the light upward 
and then horizontally to the 
screen (397)- 



24. Do not forget that one 
can do very good work by using 
an ordinary microscope in pro- 
jection. 

25. Do not try to use a hori- 
zontal microscope when one in a 
vertical position is called for. 



26. Have everything in per- 
fect order and adjustment when- 
ever an exhibition of micro- 
scopic objects is to be made. 
Haphazard work will give only 
haphazard results ( 400). 



26. Do not do haphazard 
projection. 



CH. IX] 



DO AND DO NOT IN MICRO-PROJECTION 



317 



27. For high powers like oil 
immersions, the screen distance 
must be short, the screen and 
light perfect, the room very 
dark and the spectators close 
to the screen ( 401-410). 

28. Remember the advan- 
tages of the small-carbon arc 
lamp for use on the house light- 
ing system for drawing and for 
demonstrating to a few ( 417). 

29. Use sunlight when it is 
available (419). 

30. One can do excellent 
micro-projection by home- 
assembled apparatus ( 424). 



31. For passing from micro- 
projection to lantern-slide pro- 
jection it must be remembered 
that the lantern-slide picture is 
much brighter with the same arc 
light. To avoid the great con- 
trast, one would do well to use 
a tinted glass in the magic 
lantern to soften the light as 
for opaque and lantern-slide 
projection ( 282). 

32. Study faithfully the 
"troubles" with the magic lan- 
tern in Ch. I, and in this chap- 
ter ( 435-439)- 



27. Do not try high power 
projection for a long screen dis- 
tance, a light room or a poor 
screen, or anything else not in 
accordance with the most exact- 
ing work. 

28. Do not forget the advan- 
tages of the small-carbon arc 
lamp on the house lighting 
system for drawing and demon- 
strations for a few persons. 

29. Do not neglect the most 
brilliant light, i. e., sunlight, 
when it is available. 

30. Do not refrain from micro- 
projection because you do not 
have an expensive special out- 
fit. Home-made apparatus is 
often more effective and can be 
assembled by any one. 

31. Do not forget the phy- 
siology of vision in passing from 
a dim to a brilliant light or the 
reverse. 



32. Do not expect the appra- 
atus to supply the brains. 



THE METRIC SYSTEM 



[Cn. IX 




THE METER FOR 
LENGTH 



10 CENTIMETER RULE 

THE UPPER EDGE IN MILLIMETERS, THE LOWER IN CENTIMETERS, AND HALF 

CENTIMETERS 

THE METRIC SYSTEM 
UNITS THE MOST COMMONLY USED DIVISIONS AND MULTIPLES 

Centimeter (cm.), o.oi Meter; Millimeter (mm.), o.ooi 
Meter; Micron (/*), o.ooi Millimeter; the Micron is 
the unit in Micrometry. 

Kilometer, 1000 Meters; used in measuring roads and 
other long distances. 

THE GR\M FOR f Alilli g ram ( m g-), - O1 Gram. 

W _ T ; Kilogram, 1000 Grams, used for ordinary masses, like 

\\ I- 1 GUT 

I groceries, etc. 
THE LITER FOR f Cubic Centimeter (cc.), o.ooi Liter. This is more com- 

CAPACITY j mon than the correct form, Milliliter. 

Divisions of the Units are indicated by the Latin prefixes: deci, o.i ; centi, 
o.oi; milli, o.ooi; micro, one millionth (o.oooooi) of any unit. 

Multiples are designated by the Greek prefixes; deka, 10 times; hecto, 100 
times; kilo, 1000 times; myria, 10,000 times; mega, one million (1,000,000) 
times any unit. 

TABLE OF METRIC AND ENGLISH MEASURES 

Meter (M.) (unit of length) = 100 centimeters; 

1,000 millimeters, 1,000,000 microns (/*) 39-3& inches; 3.28 feet; 

1.094 yard. 

Centimeter (cm.)=.oi meter; 10 millimeters, 

10,000 microns (/*) -3937 (I) inches 

Millimeter (mm.)=.ooi meter, .1 centimeter, 

i, ooo microns (jj.). -03937 (1^ inches 

Micron (M) = .OOI millimeter (unit of measure in 

micrometry) 000,039,37 inch 

1/25000 inch 

Liter (L.) (unit of capacity) = I, ooo cubic centi- 
meters (i quart approx.) 

Cubic Centimeter (cc.) =.oor liter (,',., cubic inch approx.) 

Gram (g.) (unit of weight) 15-43 grains. 

Kilogram (Kg.) = 1,000 grams 2.2046 (2*) pounds. 

Yard =3 feet, 36 inches 91 .44 centimeters. 

Foot = one-third yard, 12 inches 30.47 centimeters. 

Inch = 3',-, yard, ,'_>- foot 2.54 centimeters. 

.001 (, g^jj) inch 0254 millimeters =25.4 M- 

Fluid ounce = 8 fluidrachms 29.57 (3) cubic centimeters. 

Pound (LI).) (avoirdupois) = 16 ounces 453-6 grams. 

Ounce (oz.) (avoirdupois) = 437^2 grains 28.35 (30) grains. 

Ounce (oz.) (Troy or apothecaries) =480 grains . .31.10 (30) grams. 



CHAPTER X 

DRAWING AND PHOTOGRAPHY BY THE AID OF 
PROJECTION APPARATUS. 

450. Apparatus and Material for Chapter X : 

Room with electric current supply ( 453); Arc lamp and 
rheostat or other regulating device ( 462, 493) ; Water-cell ( 504) ; 
Carbons of various sizes for small and large currents ( 486, 488-9) ; 
Condenser suitable for the objects to be projected (467, 533) ; Mi- 
croscope with objectives and oculars and with a 45 degree mirror 
or a prism ( 458-459, 493) ; Photographic objectives for projecting 
the images of large objects for use with negatives or lantern slides 
( 534) ; Movable drawing surface ( 459-460) ; An opaque lantern 
( 469) ; A photographic camera with ground-glass focusing screen 
( 471); Metric measures; Transparent micrometer ( 508 + ); 
Letters on tissue paper for drawings ( 528 + ); Photographic 
paper, negatives and chemicals ( 532, 547); See also the needs 
in Ch. I, ( i). 

451. For the history of drawing with projection apparatus 
see the Appendix, with its references to literature. 

It will also be advantageous to consult the works given in Ch. I, 
2 and the catalogues of the manufacturers of projection and 
photographic apparatus. The Eastman Kodak Co. has published 
a very useful booklet on Enlarging. This deals, not with micro- 
scopic, but with the moderate enlargements up to 20 diameters with 
photographic objectives. 

DRAWING WITH PROJECTION APPARATUS 

452. The aid which projection apparatus could give for 
getting accurate drawings was recognized from the beginning; and, 
indeed, this was considered one of its most important uses. 

By the aid of projection apparatus accurate drawings can be 
made by any careful worker, although artistic perfection can be 
added only by those gifted of nature. Even for born artists it is 
helpful in getting the details of complex objects in due position and 
in correct proportion. 

319 



320 ROOM FOR DRAWING [Cn. X 

The range of possibility is great, for, by the aid of projection 
apparatus, one can draw the images produced by the objectives 
used with the magic lantern, photographic objectives, and micro- 
scopic objectives of all powers. The microscopic objectives may 
also be combined with amplifiers or with oculars for projecting the 
images to be drawn. 

Drawing with projection apparatus has the advantage over 
drawing with the camera lucida that one can see the entire specimen 
in one field. More important still, the artist can use both eyes. 
There is entire freedom of head and eyes, the image remaining 
constantly in one place, regardless of the position of the draughts- 
man. 

ROOM FOR DRAWING WITH PROJECTION APPARATUS 

453. Any room suitable for projection is also suitable for 
drawing with projection apparatus. 

Any laboratory which can be made moderately dark in the 
day time is suitable for day work; and, of course, any room is 
suitable in the evening. 

454. Special photographic and drawing room. Many labora- 
tories have one or more photographic rooms which are also used for 
drawing. These are mostly separate rooms. Sometimes they are 
adjoining a laboratory, and sometimes they are like a ticket booth 
in a large railroad station, i. e., a room within a larger room (fig. 
179). This is the plan adopted in the Wistar Institute (Anat. 
Record, 1907) and in the author's laboratory (Proc. Amer. Micr. 
Soc., 1906, p. 44-45). If these rooms are painted dull black 
within, stray light is absorbed, and it is much easier to get sharp 
pictures. In a black room the door can be left partly open and 
thus secure better ventilation. 

As the radiant gives off much heat it is an advantage to have an 
electric fan in the room if one works several hours at a time. It 
is espccialh' necessary in hot weather, summer vacations when 
teachers have time for research. 

455. A drawing room made with screens or curtains. If one 

has not a permanent room or booth in the laboratory, a fairly good 



CH. X] 



PROJECTION APPARATUS FOR DRAWING 



321 



substitute can be made by means of opaque curtains enclosing one 
corner of a room. This would be something like the early drawing 
rooms or tents used by Kepler and others for sketching landscapes 
(fig. 88, 89). It is advantageous to have the cloth curtains 
rendered fire -proof by saturating them with a solution of sodium 
tungstate, or some other fire-proofing solution (see Popular Science 
Monthly, Vol. LXXXI, 1912, p. 397). 

(Proceedings of the Amer. Assoc. Adv. Science, Vol. XLIII, 
1894, p. 119.) 




FIG. 179. PHOTOGRAPHIC AND DRAWING BOOTH (P D) IN A LARGE 

LABORATORY. 

This booth contains water and electric supply for photography and for 
projection work including drawing, printing and photo-micrography. 

PROJECTION APPARATUS FOR DRAWING 

456. The apparatus used for drawing may be the ordinary 
magic lantern (fig. 1-2), the projection microscope (fig. 121), the 
opaque lantern (fig. 92-111), or a photographic camera (fig. 117, 
217). 

457. Drawing on a vertical surface. For this, the only addi- 
tion to any of the forms of projection apparatus is a vertical draw- 
ing-board, mounted so that it may be moved to a greater or less 
distance from the apparatus to get the desired size of image. Or 
one may use a fixed wall for the drawing surface and move the 



322 PROJECTION APPARATUS FOR DRAWING [Cn. X 

apparatus back and forth to get the different sizes required. (For 
getting the picture like the object see 512). 

458. Drawing on a horizontal surface. From the earliest use 
of projection apparatus for drawing, it was the custom to draw the 
image on a vertical surface, or by means of a plane mirror to change 
the direction of the rays of light so that the image would fall on a 
horizontal surface. It was found also that when a plane mirror 




FIG. 1 80. PROJECTION MICROSCOPE FROM CHEVALIER (Planche 2). 

M Mirror reflecting the sun's rays (RR 1 , rr 1 ) to the condenser (C); from 
the condenser they pass to the substage condenser (c) and are condensed upon 
the object (o). 

L Achromatic objective. 

A Amplifier composed of a plano-convex and a double concave lens; this 
amplifier makes the rays much more divergent, i. e., BB' instead of bb 1 . 

P Right-angled prism acting as a 45 degree mirror to project the image 
down upon a horizontal surface for drawing. 

was used, the image on the horizontal surface appeared erect. 
Sometimes the mirror was placed before the objective and changed 
the direction of the rays 90 degrees (fig. 89), and sometimes it was 
used to bend the rays downward after passing through the objec- 
tive. With the microscope and magic lantern the mirror is usually 
beyond the objective (fig. 182, 193). 

Reflecting prisms have been much employed with the microscope 
instead of mirrors (fig. 180, 192). The}- have the advantage of 
giving more perfect reflection and of avoiding doubling of the 
image, as occurs with a plane mirror silvered on the back. 



CH. X] 



PROJECTION APPARATUS FOR DRAWING 



323 



459. Drawing table with attached 45 degree mirror. One of 

the simplest and most convenient arrangements for the magic 
lantern and the microscope is to have a large mirror attached to 
the drawing table. The table and mirror can then be moved 
toward or from the projection apparatus to aid in getting the 
desired magnification (fig. 182). 




FIG. 181. KORISTKA'S SIMPLE DRAWING OUTFIT. 
(From Koristka's Microscope Catalogue). 

This drawing outfit can be connected with the house lighting system. 
/ Nernst lamp for illumination. 

2 Condenser connected with the lamp-house. 

3 Stage of the microscope. 

4 Projection objective. 

5 45 mirror for reflecting the rays down upon the horizontal drawing 
surface. 

6 Horizontal drawing surface. The drawing-board slides along the axis, 
thus making it possible to vary the distance and hence to increase or diminish 
the size of the drawing at pleasure. 

When sitting down to draw, a convenient height for the table is 
76 cm. (2^ ft.). The one shown in fig. 182 has a top 100 cm. long 
and 75 cm. wide (39 x 30 inches). 

The plate glass mirror is 75 cm. long and 60 cm. wide (2^ x 2 ft.) . 
It is permanently fixed at 45 degrees inclination; and to avoid the 
sharp angle at the base of the mirror it is raised from the table 10 
to 15 cm. (4 to 6 in.). 

The mirror itself is in a strong wooden frame, and it is supported 
by vertical and horizontal pieces, as shown in figure 182. 



324 PROJECTION APPARATUS FOR DRAWING [Cn. X 




FIG. 182. DRAWING WITH PROJECTION APPARATUS AND A MOVABLE 
TABLE WITH 45 MIRROR. 

Commencing at the left : 

Supply wires to the table switch. 

From one pole of the table switch a wire extends to the binding post of the 
upper carbon of the arc lamp. 

From the other pole of the switch a wire extends to the rheostat (R) and from 
the rheostat to the binding post of the lower carbon. 

Arc lamp within the lamp-house. 

The metal lamp-house is shown as if transparent, as it was left in position 
during only a part of the time while the photograph was exposed. 

Condenser and water-cell (fig. 121). 

Stage of the microscope with stage water-cell. 

Projection microscope with objectives in the revolving nose-piece, a shield 
to stop stray light and an amplifier in the end of the large tube. 

The lamp, condenser, stage and microscope are on independent blocks and 
can be moved freely on the optical bench. The picture of the 10 centimeter 
rule under the door of the lamp-house gives the scale of the picture. 

R Adjustable rheostat. 

20-10 These numerals show the range of current which the rheostat per- 
mits. The arrow indicates the way to turn the knob to increase the current 
(see fig. 281, Ch. XIII). 

On the legs at the left is a shelf for the rheostat. 

The adjustable drawing shelf has an arrangement for moving up and down 
on metal wavs which can be attached to any table, whatever the form of the 



CH. X] PROJECTION APPARATUS FOR DRAWING 325 

legs. The supporting brackets are so jointed that the shelf can be let down 
when the large drawing table needs to be brought up close to the projection 
table. This method of moving the drawing shelf and lowering it is due to 
Dr. B. F. Kingsbury. 

As one must sit close to the table, there should be no vertical rail 
under the front edge to interfere with the knees of the artist. At 
this edge there is a strengthening piece flat against the top. On 
the other edge and at the ends are the usual vertical rails. To 
ensure the rigidity of the table, there are pieces passing across the 
ends between the legs and near the bottom, and a middle piece 
extending lengthwise between these end pieces, thus holding the 
table legs at the two ends, so that they cannot spread either side- 
wise or endwise (fig. 182). 

The legs are 6 cm. (2^ in.) square, and smooth on the lower end 
so that the table can be moved easily, or casters may be used. The 
entire table is finished in dull black and all the corners rounded. 

460. Projection table with drawing shelf. The simplest of 
all arrangements for drawing with the projection microscope and 
the magic lantern is a projection table with an adjustable shelf 
attached to the end (fig. 183, 187). For this arrangement the 
mirror or prism for reflecting the light downward must be close to 
the objective or to the end of the microscope. 

As the shelf can be raised to the level of the table top or depressed 
about 50 cm. (20 in.), it is possible to get quite a range of magnifica- 
tion from the different image distances alone, using the same objec- 
tive; but, of course, the upper range is not so great as with a 
separate drawing table. With the drawing shelf, however, one 
can get lower powers, as the image can be closer to the end of the 
objective. By using different objectives one can get all the range 
desired with either arrangement. The single table and adjustable 
shelf is, of course, much the cheaper. 

If one uses the table and drawing shelf it is necessary that the 
apparatus be movable on the optical bench, so that the objective 
ma}* be beyond the end of the table over the drawing shelf. This 
is easily accomplished with an optical bench like that shown in fig. 
158-159. In case one desires a larger drawing surface than the 



326 



PROJECTION APPARATUS FOR DRAWING [Cn. X 




FIG. 183. 



ARRANGEMENT FOR DRAWING OBJECTS THE SIZE OF LANTERN- 
SLIDES. 



The illumination can l:c by the ordinary heavy lantern-slide current, or by 
the small current of the house lighting supply. The 5 ampere current is 
sufficient for drawing. If one wishes to draw on a horizontal surface, then a 
mirror is put beyond the objective. If the drawing is on a vertical surface, as 
for wall diagrams, then the mirror is removed. 

w Supply wire cable from the outlet box (fig. 3). 

/ u. 1 Wires to the arc lamp. 

s Table switch. 

r Rheostat of the theater-dimmer type with a range of 5 to 35 amperes. 

/ Arc lamp. 

a a a Axis. 

c Condenser and water cell. 

Is Lantern-slide support. 

o Projection objective for large objects. 

m 45 mirror to reflect the light down upon the horizontal drawing shelf. 

as Adjustable drawing shelf. 

b Baseboard with track along which the, carrying blocks can be moved 
independently. 

attached shelf, a small drawing-board may be clamped to the shelf 
as shown in fig. 183. 



CH. X] 



PROJECTION APPARATUS FOR DRAWING 



327 



The size of the projection table is the same as given above ( 424) . 
A convenient size for the drawing shelf is 50 cm. (20 in.) long, and 
25 cm. (10 in.) wide. 

In fig. 183 the legs of the table are square and straight and the 
shelf slides up and down on the legs, being clamped in any desired 
position by the thumb nuts. 




FIG. 184. DR. RILEY'S ATTACHMENT TO AN ORDINARY MAGIC LANTERN 

FOR DRAWING. 

(Science, Vol. XXIX, IQOQ, p. 37-38). 

AB Mirror support. 
CD Mirror and mirror frame. 

F Clamp for fastening the mirror support in position in front of the magic 
lantern objective. 

E Drawing paper under the mirror. 



In figure 182 is shown a neat and efficient arrangement designed 
by Dr. B. F. Kingsbury, in which the shelf is hinged so that it can 
be lowered out of the way when using the drawing table with 
attached 45 degree mirror. The guides for sliding the shelf up or 
down and clamping it in any desired position, arc of metal and can 
be attached to any table whether the legs are square, tapering or 
of any other form. 



328 RADIANTS FOR DRAWING [Cn. X 

RADIANTS FOR DRAWING APPARATUS 

461. General. The best light for projection is naturally the 
best light for drawing with projection apparatus. One must 
always keep in mind that a rather dim light in a perfectly dark 
room, after one has been long enough in it to acquire twilight 
vision, may seem quite brilliant. The old observers with their 
very dim artificial lights understood this well, and did much with 
projection apparatus which at first sight would seem impossible to 
us. 

The electric arc and other brilliant artificial lights are so common 
at the present that many have come to feel that they cannot see at 
all unless the object is flooded with light. But, excepting those 
who are night-blind, that is, have poor twilight vision, much can be 
done with the Welsbach mantle light, the alco-radiant mantle 
light, etc. Even a kerosene lamp of good quality is very service- 
able, but one must always keep in mind that the dimmer the light- 
source, the darker must be the work-room, and the more care must 
be taken to avoid stray light. Too high powers should not be used 
with weak lights. For high power drawing very brilliant light is 
necessary. 

462. Arc lamp with direct current. This is, of all the 

artificial sources, the most satisfactory for drawing, as for projec- 
tion (fig. 3). With it the drawing room need not be very dark, and 
one can obtain sufficient light for the highest powers with which it 
is desirable to draw. Ordinarily a 5-10 ampere current is sufficient 
(sec also 485). If low amperages arc used the apparatus is not so 
greatly heated as with higher amperages, and furthermore the 
specimens are less liable to injury from overheating. 

The same lamp that is used for projection is suitable for drawing. 
There is some advantage in having an automatic arc lamp, then the 
artist will not have to bother about the lamp except to supply it 
with proper carbons, and to see that they arc in proper position. 
With the hand-feed arc lamps the carbons must be brought closer 
together about every 3-5 minutes. It is a convenience if the artist 



CH. X] DRAWING WITH THE MAGIC LANTERN 329 

has some sort of device, like a Hooke's jointed rod, so that the lamp 
may be adjusted without getting up (see fig. 43). 

For the arc lamp on the house circuit see Ch. Ill and 486 below. 

463. Other radiants for drawing. Any of the sources of light 
discussed in the first six chapters can be used for drawing. One 
must use the precautions given in those chapters for getting a good 
screen image by a proper alignment and separation of the elements 
of the apparatus, and by suiting the darkness of the room to the 
light. 

DRAWING WITH THE MAGIC LANTERN 

464. Drawing wall diagrams. The simplest form of projec- 
tion for drawing is with the magic lantern. With it the preparation 
of wall diagrams is very easy (fig. 185). 

If one has a lantern slide of the picture or object to be drawn it is 
put into the lantern as for ordinary projection. The drawing- 
board is then arranged at a distance to give the desired size, and 
then all the lines traced with a crayon, a brush or a coarse pen. One 
can use water colors or paints. For the black nothing is better 
than India ink. 

If one has a smooth wall to which the drawing paper or cloth can 
be fastened, then the lantern can be moved closer or farther away 
to get the desired size. 

If one has no lantern slide, then a negative may be made of the 
subject to be drawn, and the negative used in the lantern instead of 
the lantern slide. The negative should not be too dense or the 
lines will not come out clearly. 

For making negatives to draw from, it is advantageous to use 
lantern-slide dry plates. These will be of the right size for the 
lantern and are more transparent than ordinary negatives. 

For lettering diagrams nothing is more convenient than the large 
rubber type found in sets used in advertising and sign making. 

465. Getting the desired size. Any desired size may be 
obtained by varying the distance between the drawing surface and 
the projection objective. Either the lantern or the drawing surface 
or both must be movable. 



330 



DRAWING WITH THE MAGIC LANTERN 



[CH. X 



The size of the drawing can be varied without moving the lantern 
or the drawing surface by using an objective of longer focus for a 
smaller diagram, or of a shorter focus for a larger diagram (see also 
57)- 

466. Use of the magic lantern for small drawings. It fre- 
quently happens that a small drawing of some large object is 
needed for publication. This may be some natural object or a 
piece of apparatus. The object or piece of apparatus is placed in a 
good light and a small negative made on a lantern-slide plate, being 
careful not to make the negative too dense. After this is dry, it 
can be put into the lantern-slide carrier and projected upon the 
drawing paper, and the outlines accurately traced. Then with a 
pen and India ink one can ink in the lines and add any necessary 
shading free-hand, having the object or piece of apparatus in view 
so that it can be accuratclv done. The exact magnification or 



Condenser 



Arc Lamp 




KS 



FIG. 185. SIMPLEST FORM OF MAGIC LANTERN WITH ARC LIGHT FOR USE 

IN DRAWING. 

SW Supply wires. 

So K Socket with its key switch. 

S P Separable attachment plug. 

LW Wires extending from the cap of the plug to the knife switch. 

KS Knife switch for turning the current on and off. 

Rheostat The balancing device for regulating the current. 

Arc Lamp The arc lamp with right-angled carbons. 

Condenser The two-lens condenser with the first (i) and the second (2) 
elements. 

LS Position of the lantern slide or other large object. 

Objective with r, its center. 

Axis Axis The principal optic axis of the condenser and the objective. 
The radiant must be centered on this axis. 

Image Screen The drawing surface on which the image is projected. 



CH. X] DRAWING WITH THE MAGIC LANTERN 331 

reduction of the picture can be determined by photographing a 
metric or other measure (fig. 178) on the same plate with the 
object or piece of apparatus. 

467. Size of condenser necessary for making drawings. 

When lantern slides, or negatives made on lantern-slide plates or 
other plates of that size are used, the condenser of any magic lan- 
tern will answer. Sometimes, however, it is desired to make dia- 
grams or drawings from negatives of larger size. There are two 
ways of accomplishing this: 

(1) A lantern slide can be made from the large negative by the 
aid of a photographic objective as described in Ch. VIII, 329. 
This can then be used in the ordirary lantern. 

(2) If the large negative is to be used direct, then the condenser 
of the magic lantern must be of sufficient size to illuminate the 
negative. That is, the condenser must have a diameter a little 
greater than the diagonal of the negative to be illuminated and 
drawn (see fig. 114). 

468. Drawing on a horizontal surface by the aid of the magic 
lantern. This is easily accomplished by using a 45 degree mirror 
or a prism beyond the objective (fig. 192). 

One must be careful to put the negative or lantern slide in the 
carrier in such a way as to give an erect image ( 512). 

If the negative or lantern slide or other object is too dense, so 
that the light is relatively dim, the image will be duplicated when a 
mirror silvered on the back is used, therefore, one must use a prism 
or a mirror silvered on the face for these dark objects. For very 
transparent objects the image appears single even with a mirror 
silvered on the back, the silver image being so much brighter than 
the glass image that the latter does not show. 

One can use the magic lantern and separate table with a 45 degree 
mirror (fig. 182) or the mirror can be fastened to the projection 
table as in Dr. Riley's device (fig. 184) or the mirror may be close 
to the objective, and the adjustable drawing shelf used (fig. 
183). 



332 DRAWING WITH THE EPISCOPE [Cn. X 

DRAWING WITH THE EPISCOPE OR REFLECTING LANTERN 

469. If one has access to a lantern for opaque objects (Chap. 
VII), diagrams may be made from pictures in books and from suit- 
able objects without the trouble of making a negative or a lantern 
slide. The object is put in position in the reflecting lantern and its 
image thrown upon the drawing surface. It can then be traced 
as for a lantern-slide image, and the details, shading and lettering 
added as described for diagrams made from lantern slides or from 
negatives (464). 

470. Drawing on a horizontal surface by the aid of the opaque 
lantern. If the apparatus is suitably arranged, the mirror will 
throw the image downward upon a horizontal surface instead of out 
horizontally. Then the tracing can be made as for a lantern slide 
( 468). There is one difficulty with the reflecting lantern in mak- 
ing drawings. If the object to be drawn is of some thickness, only 
a part of it will be in focus at any one time, hence it is not easy to 
get the parts in true perspective. (For erect images see 514). 

If one makes a small negative with a good objective, the perspec- 
tive will be good and all the parts will be in focus. 

When this negative is projected upon the drawing surface with an 
ordinary lantern, all the parts of the image will be in focus. 

If one wishes drawings of flat objects, pictures in books, etc., the 
opaque lantern answers admirably, but heavy currents are re- 
quired, and it is not so safe for inexperienced persons as the magic 
lantern with a small current and a negative or a lantern slide (see 
further in Ch. VII, 290). 

DRAWING WITH A PHOTOGRAPHIC CAMERA 

471. The drawing of enlargements or reductions of opaque 
objects with the photographic camera has been much practised. 
The object is put in a good light and arranged to show the desired 
aspect, then a photographic camera is directed toward it, and the 
bellows lengthened or shortened until the picture on the ground- 
glass focusing screen is of the desired size. Then the plate holder 
with a clear glass or a focusing screen of clear glass is used and over 



CH. X] DRAWING WITH CAMERA AND MICROSCOPE 333 

it some tracing paper. By covering the head with a focusing cloth 
to shut out the surrounding light, one can trace the outlines of the 
object on the tracing paper, and transfer these to ordinary drawing 
paper, and proceed to ink them in and give the shading necessary 
free-hand. 

With the magic lantern or with the opaque lantern the image is 
projected upon the drawing surface and regular drawing paper can 
be used to make the original pencil tracing upon, but with the 
camera one must use translucent paper for the tracing and then 
transfer it to the drawing paper. (To get an erect image with 
translucent paper see 519). 

DRAWING WITH THE PROJECTION MICROSCOPE 

472. Range of objects. For drawing as for projection it is 
exceedingly desirable that the projection microscope should enable 
the investigator to commence where the magic lantern leaves off, 
and to carry the \vork to its utmost possibilities; that is, begin- 
ning with large specimens of 50 to 60 mm. (2 in.) in diameter re- 
quiring low objectives, and going on from this to the smallest 
objects visible and using the oil immersion objective at the other 
extreme. 

To realize this ideal possibility one must have available for 
drawing some such outfit as that described in Ch. IX for projec- 
tion ; and in addition suitable arrangements for reflecting the image 
down upon a horizontal drawing surface. Fortunately, the addi- 
tions are relatively simple and inexpensive. 

Finally, for the widest usefulness in drawing there must be the 
possibility of using the ordinary house electric lighting system for 
an electric lamp with small carbons (see 486). 

473. Drawing large objects with low powers. For this it is 
necessary to have a stage with a large opening (fig. 134), and the 
objective must be mounted in a shield with no tube at all (fig. 138), 
or the tube must be short and of large diameter, so that the field is 
not restricted (fig. 137). Finally, there must be some means of 
increasing or diminishing the distance between the objective and 
the drawing surface to get the desired magnification. 



334 DRAWING WITH PROJECTION MICROSCOPE [Cn. X 



h 




FIG. 1 86. 



APPARATUS FOR DRAWING WITH THE MICROSCOPE WITHOUT 
AN OCULAR OR SUBSTAGE CONDENSER. 



The arc'lamp is Mr. Albert T. Thompson's automatic lamp for direct current, 
5-25 amperes. This is the first automatic arc lamp for right-angled carbons. 

By means of the optical bench carrying all the apparatus, the different 
parts are pulled forward so that the microscope tube and mirror project over 
the drawing shelf. This is adjustable up and down for varying the magnifi- 
cation. 

The stage of the microscope (st) is independent and contains a large glass 
water-cell against which the specimen rests. It conducts away the heat from 
the specimen. 

a a a Optic axis. 

b Optical bench with track. 

c Triple condenser with water-cell. 

/ Thompson's automatic arc lamp for 5-25 amperes direct current. 

m Microscope without ocular. The 45 mirror reflects the light down upon 
the drawing surface. 

r Adjustable rheostat. 

s Double-pole knife switch (table switch). 

st Stage with the stage water-cell for cooling the specimen. The stage is 
entirely separate from the microscope. 

sh Shield 25 cm. in diameter to stop any stray light from the stage of the 
microscope. 

wi Double cable supplying the electric current to the apparatus. 

W2 Flexible cables from the switch to the lamp. 



CH. X] DRAWING WITH PROJECTION MICROSCOPE 335 

474. Varying the drawing distance. The drawing distance 
is easily .varied by means of a movable table like that figured (fig. 
182), or by an adjustable shelf attached to the projection table (fig. 

183)- 

Another way of varying the size of the drawing is to use higher or 
lower objectives, the drawing distance remaining the same (see 
57)- 

475. Lighting the object. For large objects and low powers 
the best way to illuminate the object is to use the main condenser 
only and to put the object in the cone of light where it is fully 
illuminated (fig. 132). If the drawing shelf is used this will involve 
moving the lamp and condenser toward the drawing-board ; for the 
microscope must be beyond the end of the table, so that the image 
can be thrown down on the shelf, (fig. 186). The change in posi- 
tion of any part or parts is, of course, very easy with an optical 
bench (fig. 158-159). 

476. Drawings with objectives of 16, 12, 10, and 8 mm. 

With objectives of this range without an ocular, one can draw 
objects varying from 5 to 2 mm. in diameter. For lighting, use the 
large condenser and focus the image of the crater on the hood of the 
objective (fig. 140), and then push the stage up toward the objec- 
tive until the object is in focus, finishing the fine focusing with the 
micrometer screw of the microscope. 

DRAWING WITH THE PROJECTION MICROSCOPE, INCLUDING AN 
OCULAR AND A SUBSTAGE CONDENSER. 

477. Drawing fine details with high powers (8 to 2 mm. focus) . 

As pointed out for the projection of images showing fine details 
( 401), it is necessary to use a substage condenser to get the neces- 
sary aperture of the lighting beam, and to use an ocular to com- 
pensate for objective defects. If one uses a water or an oil immer- 
sion objective the proper immersion fluid must be used between the 
cover-glass and the objective, as in ordinary microscopic work. 

478. Parallelizing the converging beam of light. The sub- 
stage condenser used for ordinary observation is designed for ap- 



336 



[Cn. X 




FIG. 187. AN ORDINARY MICROSCOPE USED WITH THE LAMP AND 
CONDENSER OF A MAGIC LANTERN FOR DRAWING OR PROJECTION. 

W The supply cable from the outlet box (fig. 3). 

s The table, knife switch. 

r Rheostat of the theater-dimmer type. 

/ The automatic arc lamp. This is the three-wire automatic arc lamp of 
the Bausch & Lomb Optical Company for 5-25 amperes. (For the method 
of connecting the wires see 704). 

c Triple condenser with water-cell. 

a a Principal optic axis. 

p The concave parallelizing lens to render the converging cone from the 
condenser parallel or nearly so before entering the sub stage condenser of the 
microscope. 

m The microscope in a horizontal position. If it is to be used for drawing 
there must be a prism or mirror beyond the ocular to reflect the light down on 
the drawing shelf. 

b Baseboard with track serving as an optical bench. 

a s Adjustable drawing shelf on the front of the projection table. 

proximately parallel light (fig. 150 A. B.), hence it is necessary to 
render the converging cone of light from the main condenser 
approximately parallel. This is most easily accomplished by using 



CH. X] DRAWING WITH HIGH POWERS 337 

a plano-concave or double-concave lens. This is mounted in a 
fork-like holder and is set in the socket for the mirror stem of the 
microscope (fig. 152, 187). Then the microscope is pushed up 
toward the condenser until the parallel beam is of sufficient diam- 
eter to fill the substage condenser. The substage condenser 
diaphragm is opened to its full extent. 

479. Concave lens to be employed. This depends upon the 
focus of the main condenser. If the focus is about 15 cm. (6 in.), use 
a concave lens of -16 to -20 diopters ( 356). If the main condenser 
has a focus of 20 to 40 cm. (8 to 16 in.), use a concave lens of -8 to 
-12 diopters. The longer the focus of the main condenser the. 
shallower can be the concavity of the parallelizing lens. Indeed, 
for objectives of 16, 12, 10, and 8 mm. focus a condenser lens of 25 
to 38 cm. (10 to 15 inches) focus gives very good results, when 
the substage condenser is used without any parallelizing lens 
(fig. 154). 

480. Position of the substage condenser; opening of the 
condenser diaphragm. As pointed out in Ch. IX ( 407), the posi- 
tion of the substage condenser must be very precisely determined 
for different objectives and for different thickness of slides. 

To begin with, the substage condenser diaphragm is opened to 
its full extent. Then in each case one must get the sharpest possi- 
ble image by getting the best position of the substage condenser, 
and closing the diaphragm more or less. As a general statement, 
the diaphragm should be considerably wider open for drawing 
than for ordinary observation. 

481. Oculars to employ for drawing. Those of X2, X3, X4, x6, 

x8, and xi2 may be used. Naturally, the lower and medium 
powers give the more brilliant images as for direct observation, 
One will rarely need to use an ocular higher than x8. 

482. Mirror or prism for reflecting the image-forming rays 
down upon the drawing surface. For high power drawing it is 
better to have the reflecting mirror or prism close to the ocular 
(fig. 192) rather than to have it distant, as with the drawing table 
in figure 182. 



338 DRAWING WITH HIGH POWERS [Cn. X 

If a mirror is used it must be a perfect one and preferably slivered 
on the face to avoid duplicating the images. If it is silvered on the 
back the glass must be thin. A totally reflecting prism is best, 
but it is somewhat expensive, costing about twice as much as the 
mirror. 

483. Avoidance of distortion. Whichever is used for reflect- 
ing, it should be fitted with a stop so that it will be at 45 degrees 
with the main axis, then the image-forming rays will be reflected 
directly downward and the image will not be distorted, provided 
of course, that the mirror or prism is directly above the drawing 
surface. If it were turned over to one side more or less, the image 
would be correspondingly distorted. 

It is a good plan for one to become familiar with the distortions 
possible in drawing. For example, if the mirror or prism is not at 
45 degrees with a horizontal microscope (fig. 182, 193), the spot of 
light on the drawing surface will not be circular but elliptic, the axis 
of the ellipse being parallel with the optic axis of the microscope. 
If the prism or mirror is not directly above, but turned to one side, 
then the spot of light will be elliptic and projected to one side of the 
axis of the microscope. If one is familiar with the possible dis- 
tortions it will be easy to detect them ; then they can be corrected. 
Naturally, a drawing should be accurate when finished. 

484. Specimens suitable for drawing with high powers. Any 

object suitable for projection can have its image projected upon a 
drawing surface (see also 410). 

485. Amount of electric current required for drawing. If one 
has a direct current, 5 to 10 amperes will be sufficient for all draw- 
ing purposes. The specimens must usually be left for a consider- 
able time in the focus or near the focus of the light beam, and hence 
are liable to overheating. The lower the amperage the less the 
danger from the overheating. Then it is not good for the eyes 
of the artist to have the light on the drawing surface too dazzling. 

With alternating current, 6 to 15 amperes usually suffice. 

Here, as in all other projection, skill is of more account than 
overwhelming electric currents. 



CH. X] 



DRAWING WITH HOUSE CURRENT 



339 



PROJECTION DRAWING APPARATUS WITH THE RADIANT 
CONNECTED WITH THE HOUSE LIGHTING SYSTEM. 

486. General Statement. As shown in Chapter III (fig. 
41-43), the arc lamps using small, cored carbons (6 to 8 mm. in 
diameter) and drawing from three to six amperes may be connected 
with any socket for an incandescent bulb of the house lighting 
system. The light so obtained is more powerful than the usual 
lime light. The carbons being small, the light approaches closely 
to the ideal point source. Consequently for all projection pur- 
poses, including drawing, this form of arc light is of the greatest 
importance and utility. Of course, for projection in a large hall 
it is insufficient, but for the relatively small screen pictures needed 
in drawing and for small classes, the results are very satisfactory. 

487. Wiring, rheostat and connections for the arc lamp 
attached to the house lighting system. This is shown in fig. 188- 
189 and described in 128-135. Remember and practice the 
advice given about turning the current on and off ( 133), and the 
possibility of short circuiting and burning out the incandescent 
bulb socket. Never use an arc lamp without a suitable rheostat 
or inductor. (See 129, also i28a for fuses on the house system). 




FIG. 1 88. WIRING AND CONNECTIONS OF THE ARC LAMP USED ON THE 
HOUSE LIGHTING SYSTEM (See fig. 45). 



340 



DRAWING WITH HOUSE CURRENT 



[Cn. X 




FIG. 189. SMALL ARC LAMP FOR DRAWING. 

Commencing at the left : 

Wi Supply wires 

So Lamp socket. 

K Key switch in the socket. 

Sp Separable attachment plug. 

W2 Wires to the arc lamp. 

W3 Wire to the binding post of the upper carbon. 

W4 Wire to the rheostat (R) and from the rheostat to the binding post 
of the lower carbon. 

A Support of the arc lamp; the lamp can be raised or lowered on this 
support. 

F Feeding screws for the carbons. 

H C, V C The horizontal and the vertical carbons. 

In In Insulation between the carbon holders and the remainder of the 
lamp. This prevents the current from taking any path away from the carbons. 

Ch Chimney over the arc. 

T C The tube and the condenser in the movable inner tube. The con- 
denser is at its focal distance from the crater, and therefore the rays are made 
parallel. 



CH. X] DRAWING WITH HOUSE CURRENT 341 

Sh Shield to stop stray light and to aid in centering. 

C Carbons with alternating current. They are of the same size. 

D Carbons with direct current. The upper one is 8 mm. and the lower one 
6 mm. in diameter. 

E Shield or disc at the end of the condenser tube showing the opening of 
the condenser (C) and the spot of light at the right. 

488. Arc lamp and small carbons. The form of arc lamp to 
use on the house circuit is not of particular importance. It may 
be very conveniently one of the small lamps shown in fig. 41-44, 
201, 205, or it can be an ordinary arc lamp for greater currents, 
but supplied with long clamping screws, bushings or adapters for 
the small carbons ( 127). The small lamps are generally of the 
hand-feed type and move the upper and the lower carbons equally. 

489. Size of carbons for direct current. A. The carbons 
found useful for direct current are as follows, all being of the soft- 
cored variety: 

(1) Upper or positive carbon 7 mm. in diameter, lower or nega- 
tive carbon 5 mm. 

(2) Upper carbon 8 mm., lower 6 mm. 

(3) Upper carbon n mm., lower 8 mm. 

B. The carbons for alternating current with an equal feed for 
the upper and the lower carbon, should be of the same size, and this 
size should not exceed 8 mm. in diameter for 5 to 6 amperes. If 
only three or four amperes are used, then it is better to have carbons 
not greater than 6 mm. in diameter. 

490. Reason for using small carbons. In order to have the 
light steady and thus have the field continuously bright, the entire 
end of the upper carbon should be white hot. 

If the carbon is so large that the crater covers only a part of the 
tip, the crater will wander about on the end of the carbon. Every 
change in the position of the crater changes the direction of the 
light beam. While the crater is in one position the entire field of a 
high power objective may be brilliantly illuminated; if the crater 
wanders to a new position, the field will be only partly or not at all 
illuminated. In such a case, one must constantly change the posi- 
tion of the mirror of the microscope to keep the field bright. If, 
however, the crater is nearly as large as the end of the carbon, it 



342 



DRAWING WITH HOUSE CURRENT 



[Cn. X 



will wander but little, if at all, and the light will be more con- 
stant. 

491. Feeding the carbons together. If one has an alternat- 
ing current to work with, the small arc lamp will burn about 10 
minutes with 8 mm. carbons before going out. With the right- 
angle position the carbon giving the light remains constantly in 
the axis. With inclined carbons, it rises constantly above the axis. 
The carbons with the right-angle arc should be fed about every 
five to seven minutes to insure the best light. 




FIG. 190. SIDE AND FRONT VIEW OF SMALL CARBONS WITH FIVE 
AMPERES OF DIRECT CURRENT (Natural Size). Compare fig. 191. 




FIG. 191. SIDE AND FRONT VIEW OF SMALL CARBONS WITH FIVE AMPERES 
OF ALTERNATING CURRENT (Natural Size). 

The crater is much smaller than with direct current (fig. 190). 



CH. X] DRAWING WITH HOUSE CURRENT 343 

If direct current is used, the lamp will burn for about six 
minutes and the carbons should be fed together every three to five 
minutes. (See fig. 205). 



CONDENSER, STAGE AND MICROSCOPE FOR DRAWING WITH THE 
HOUSE LIGHTING SYSTEM 

492. Drawing outfit. If one has a drawing outfit consisting 
of the projection apparatus shown in figure 182, all that is necessary 
to do is to place the arc lamp with its small carbons in the lamp- 
house and arrange it exactly as for projection. 

The procedure is precisely as described above for the ordinary 
arc lamp on the usual special lantern lighting system (Ch. IX). 

493. Small Current Outfit. This consists of an arc lamp 
using small carbons (6 to 8 mm. in diameter) and a rheostat or an 
inductor (fig. 197) not allowing over 5 to 6 amperes of current to 
flow. Instead of the usual large condenser (fig. 1 2 1) , a small, single, 
convex lens is used. This is of 70 to 100 mm. (3 to 4 in.) focus, 
and 37 to 50 mm. (i^ to 2 in.) in diameter, and is placed in a tube 
extending straight out from the upper carbon. Usually, also, the 
lens is in a sliding tube, so that it may be varied in distance from 
the source of light. If it is at its focal distance from the light, the 
beam will be approximately parallel (fig. 189); if farther from the 
light, the beam will be converging. 

494. Method of using the lamp with a special condenser. 

There are three methods of using this arc lamp and special con- 
denser : 

(1) The lamp can be put in line with the drawing microscope 
and a converging beam thrown directly on the specimen as for the 
large apparatus (fig. 132), the mirror and sometimes the substage 
condenser having been removed or turned aside. 

(2) The mirror is removed from the microscope, but the sub- 
stage condenser is left in position, and a parallel beam of light 
thrown directly into the substage condenser along the optic axis 

(fig. 20lA). 



344 

Substage 
Condenser 



DRAWING WITH HOUSE CURRENT 



[Cn. X 



Microscope _ 

Objective '' ^ Ocular 




FIG. 192. 



DIAGRAM OF THE MICROSCOPE ARRANGED FOR DRAWING ON A 
HORIZONTAL SURFACE. 



The light is from an arc lamp supplied by the house current (fig. 188, 189). 

A right-angled prism is used to reflect the rays down upon the drawing sur- 
face. 

The designations are self explanatory except in the ocular r i means the real 
image formed by the objective and field lens (see fig. 207). 

The adjustment screw heads at the side of the microscope are: 

/ a Fine adjustment. 

c a Coarse adjustment. 

H The handle in the pillar for carrying the microscope. 



(3) From the difficulty of getting the small lamp and condenser 
in the optic axis without the use of an optical bench it has been 
found much easier to get the light upon the specimen and through 
the microscope by placing the arc lamp and its condenser at right 
angles to the microscope, and to use the regular microscope mirror 
for reflecting the beam through the substage condenser (fig. 193). 
If the substage condenser is not used, the mirror reflects the beam 
directly on the specimen, as for low power projection. 



CH. X] DRAWING WITH HOUSE CURRENT 345 

495. Microscope. Any modern microscope with a good sub- 
stage condenser can be used, provided it is supplied with a flexible 
pillar, so that the tube can be made horizontal; and provided also, 
that the fine adjusting mechanism will work when the tube is 
horizontal. 

There must be a prism or mirror beyond the ocular to reflect the 
image-forming rays downward upon the drawing surface (fig. 192). 

The discussion of avoidance of distortion, the proper objectives, 
oculars, etc., to use, which was given in the earlier part of this 
chapter apply here ( 452, 483). 

496. Position of the microscope for drawing. In the drawing 
outfits thus far devised, the microscope is placed in one of the 
following positions : 

(1) In an inverted position with the objective pointing directly 
upward (as in the large Edinger apparatus, fig. 202). 

(2) Inclined at 45 degrees (as in the small Edinger apparatus, 
fig. 204). 

(3) In a horizontal position (fig. 192). 

With the microscope in an inverted, vertical position, there 
should be no distortion of the image if the drawing surface is 
horizontal. 

With the inclined microscope, the mirror used must be so inclined 
that it throws the image directly down upon the horizontal drawing 
surface, or the image will be distorted. It is not easy to tell just 
the inclination of the microscope, and therefore, the exact inclina- 
tion to give the mirror, to make the axial ray perpendicular to the 
drawing surface. In the small Edinger apparatus (fig. 204), the 
directions are to make the inclination of the microscope 45 degrees 
and the inclination of the mirror 22^ degrees. This arrangement 
will give a correct image. One may need to use a protractor to 
make sure that the inclination of the microscope and mirror are 
exactly correct. 

With the horizontal microscope, the mirror or prism is so 
arranged that it is fixed at 45 degrees and therefore if put directly 
over the ocular of the horizontal microscope, will reflect the light 
perpendicularly upon the drawing surface, thus avoiding distortion 
(see 483). 



346 



[On. X 



With the horizontal microscope, unless one uses a table with a 
drawing shelf (fig. 187), the microscope must be raised on a block 
or support of some kind and clamped to the block so that it will be 
rigid (fig. 193-194). A convenient height is 250 mm. (10 inches). 

To vary the magnification slightly, the distance can be made 
greater by using an additional block, or it may be made less by 
raising the drawing surface. For a very convenient arrangement 
for changing the elevation of the microscope see fig. 198, 2oiC. 

For obtaining the scale or magnification of the drawing see 
508-510. 

497. Getting the light through the horizontal microscope 
with the plane mirror. The simplest method is to place the lamp 




FIG. 193. DRAWING WITH THE MICROSCOPE IN A DARK ROOM. 



In the arrangement here shown the light is from a small arc lamp drawing 
current from the house lighting system. 

The supply cable and the lamp socket are shown, then the separable attach- 
ment plug and the supply wires with the rheostat inserted along one wire (fig. 
1 88). 

The arc lamp is at the level of the microscope mirror and at right angles with 
the microscope axis. The light from the arc lamp is reflected up to the sub- 
stage condenser by the mirror and passes on through the specimen and micro- 
scope as shown in fig. 192. 

The shield between the lamp and drawing surface is to keep stray light from 
reaching the drawing surface. The shield is represented as transparent. It 
was left in place during only a part of the time of the exposure in making the 
photographic negative. 



CH. X] DRAWING WITH HOUSE CURRENT 347 

and the microscope at right angles. Use a level (fig. 160) and make 
sure that the condenser tube is horizontal, and the axis of the con- 
denser at the same level as the center of the mirror. Place a disc 
of blackened asbestos or tin of about 12.5 cm. (5 in.) in diameter 
just behind the condenser as shown in fig. 193-198. This is easily 
done by making a hole of the proper size in the disc to go over the 
condenser tube (fig. 195). If now the current is turned on and the 
arc established the light will extend from the condenser to the plane 
mirror and be reflected by it, if it is set at 45 degrees, up into the 
substage condenser. From the lower face of the substage con- 
denser a part of the light is reflected back to the mirror and from 
the mirror back toward the lamp, and is received by the black disc 
over the condenser tube. The mirror should then be turned until 
the spot of light enters the lamp condenser. The mirror will then 
be in position to reflect the light along the optic axis of the micro- 
scope. 

If the microscope is in focus on the object, the light will traverse 
the objective and ocular and be reflected down upon the drawing 
surface by the mirror or prism beyond the ocular. 

By changing the mirror slightly while watching the circle of light 
on the drawing surface, the best illumination can be obtained. 

498. Getting the light through the microscope with the con- 
cave mirror. One proceeds exactly as described above, only the 
light reflected back to the black disc on the lamp condenser tube 
will be a crescent instead of a circle. The middle part of the cres- 
cent can be reflected into the lamp condenser and then the light will 
pass through the microscope and be reflected down upon the draw- 
ing paper, provided the microscope and the arc lamp are at right 
angles and at the proper level. 

499. Substage condenser. Use the substage condenser with 
objectives of 16, 12, 10, 8, 6, 4, and 2 mm. focus. For objectives 
lower than the 16 mm. the substage condenser is turned aside. 

With different objectives and slides of different thickness the 
substage condenser is changed somewhat in position to get the best 
light on the object and to light the entire field. 



348 DRAWING WITH HOUSE CURRENT [Cn. X 

To start with, the substage condenser diaphragm should be 
opened widely. In some cases the picture can be made sharper by 
afterward closing the diaphragm somewhat. 

For drawing, a skillful use of the substage condenser is very 
important. One must be more precise in its use than in ordinary 
microscopic observation. 




FIG. 194. DRAWING WITH A MICROSCOPE WITH THE ARC LAMP AT RIGHT 

ANGLES. 

In this picture a prism is placed beyond the ocular to reflect the light down- 
ward (fig. 192). The arc lamp is on the back side of the microscope with the 
condenser facing the mirror. The spot of light on the shield or disc above the 
lamp shows that the light is not centered along the axis of the microscope. 
The mirror must be turned slightly until the light reflected back from the 
substage condenser and microscope mirror enters the condenser tube of the 
arc lamp (see fig. 195). 

500. Plane mirror and substage condenser. Use the plane 
mirror and substage condenser for all objectives of 12, 10, 8, 6, 4, 
and 2 mm. equivalent focus. 

501. Concave mirror and substage condenser. For the 16 to 

1 8 mm. focus objectives use the substage condenser with the con- 
cave mirror. It may also be necessary to separate the condenser 
somewhat from the preparation to light the entire field. 



CH. X] DRAWING WITH HOUSE CURRENT 349 

502. Concave mirror without a substage condenser. For 

objectives of 20, 25, 30, 35, 40 and 50 mm. focus use the concave 
mirror without a substage condenser. 

503. Immersion objective. -For immersion objectives used 
in drawing do not forget to use the proper immersion liquid 
between the cover-glass and the objective ; cedar oil for the oil 
immersions, and distilled water for the water immersions. 




FIG. 195. SHIELD AT THE END OF THE ARC LAMP CONDENSER TUBE TO 
AID IN CENTERING THE LIGHT. 

This disc is of blackened sheet iron, asbestos or cardboard and is 125 to 150 
mm. in diameter. It is placed at the end of the lamp condenser tube. If the 
light is centered, then that reflected back from the substage condenser and 
microscope mirror will enter the lamp condenser (C). If the light is not cen- 
tered there will be a round spot of light somewhere outside the lamp condenser. 
In that case the mirror must be turned slightly until the reflected light enters 
the lamp condenser. 

If the plane mirror is used the spot of light will be nearly circular; with the 
concave mirror it will be crescentic. 

C The lamp condenser. 

I Spot of light outside the condenser showing that the light is off the center. 



350 



DRAWING WITH HOUSE CURRENT 
AVOIDANCE OF HEAT 



[On. X 



504. When the small currents from the house circuit are used 
the heat is not great enough to injure most specimens mounted in 
balsam. For live objects and objects mounted in glycerin or 
glycerin jelly, etc., it would be wise to place a water-cell in the beam 
before it reaches the microscope (see 364, 3Q4a, fig. 206). 




FIG. 196. 



DRAWING OUTFIT FOR THE HOUSE LIGHTING SYSTEM WITH A 
BLACK CLOTH TENT OVER THE MICROSCOPE. 



This arrangement answers well for a moderately lighted room. Naturally 
the opening for drawing should face toward some dark furniture or the dark 
side of the room, not toward a window. 

5 Separable cap to attach to the separable plug in a socket of the house 
lighting system. 

w r One of the supply wires cut and inserted into the binding posts of the 
rheostat (see fig. 188). 

/ Small arc lamp for supplying the illumination. It is at the level of the 
mirror and at right angles to the microscope. 

m Mirror of the microscope. 

t Cloth tent over the microscope. It appears semi-transparent as it was 
left in position during but part of the time when the photograph was 
taken. 



CH. X] LIGHT SHIELD FOR DRAWING 351 

SHIELDING THE DRAWING SURFACE FROM STRAY LIGHT 

505. Shield for working in a dark room. If one works in a 
dark room all that is necessary to screen the drawing from stray 
light from the arc lamp, when the lamp is at right angles to the 
microscope, is a blackened cardboard shield (fig. 193). 

If the lamp is in line with the microscope, it will be necessary to 
put a shield with a perforation for the light beam either before the 
beam reaches the microscope, or it may be put over the tube of the 
microscope so that it will shield the drawing surface. 

506. Drawing in a light room. If this is necessary one should 
get in as shaded a part of the room as possible. To screen the 
drawing surface there are two ways : 

(1) There may be a cloth for enclosing the drawing surface and 
the head of the artist. This is like the plan used in focusing a 
photographic camera (fig. 204). 

(2) By means of cardboard, or of a wire frame and cloth cur- 
tains, a box or tent is built around the drawing surface enclosing 
also the microscope tube. The end of the box next the draughts- 
man is open sufficiently for him to see the image (fig. 196, 198). 
The drawing surface should look toward some dark furniture 
or a dark or shaded part of the room, and except for the most 
exacting work the surface will be sufficiently shaded. For the most 
exacting work, and for the greatest freedom from accessories, the 
evening or a dark room in the daytime offers the best facilities 
( 453)- 

How TO GET ANY DESIRED MAGNIFICATION IN A DRAWING 

507. The magnification can be varied by any of the following 
ways, or two or more of the ways may be combined. 

(a) By using a higher or lower objective. 

(b) By using an amplifier, of greater or less power, with the 
objective. 

(c) By using a higher or lower ocular with the objective. 

(d) By changing the distance of the drawing surface; the 
farther it is away in any given case the larger will be the image, and 
the nearer it is the smaller the image ( 5ioa). 



352 



MAGNIFICATION OF DRAWINGS 



[Cn. X 




FIG. 197. DRAWING OUTFIT FOR THE HOUSE LIGHTING SYSTEM, USING 
AN INDUCTOR INSTEAD OF A RHEOSTAT (fig. 193). 

Commencing at the left : 

The supply wires to the lamp socket, and the supply wires from the separable 
attachment plug to the arc lamp. 

One of the supply wires is connected directly with the arc lamp and one is 
cut and the two cut ends connected with the two poles of the inductor exactly 
as with a rheostat (fig. 188), and from the inductor the wire is continued to one 
of the binding posts of the arc lamp. 

The inductor is only for alternating current ( 736). The amperage can be 
varied by sliding the soft iron core in and out of the coil. The more the core 
is inserted the greater the inductance and hence the less the amount of current 
that is allowed to flow. 

As shown in the picture, the core is only partly inserted and a medium 
current is allowed to flow. 

If one uses alternating current this is a much more economical method of 
controlling the current than a rheostat (see 736 + ) and a steadier light is 
produced. 

The two most common changes are: (i) Using a higher or 
lower objective; and (2) Changing the distance of the drawing 
surface. 



CH. X] MAGNIFICATION OF DRAWINGS 353 

For slight variations in size the change in distance is by far the 
best and easiest change to make. 

If one has a drawing table (fig. 182) it is very simple to push it 
farther from or closer to the projection apparatus. 

If the drawing shelf is used it can be raised or lowered (fig. 183). 

If the simple apparatus is used on an ordinary table the entire 
microscope can be raised for higher magnifications or for lower 
magnifications the drawing surface can be raised to bring it closer 
to the microscope tube (fig. 193-204), or the microscope can be 
lowered on its adjustable support (fig. 198). 

DETERMINATION OF THE MAGNIFICATION OF A DRAWING 

508. For getting the magnification it is necessary to use for 
an object a transparent micrometer with known divisions upon it. 
For most of the work done a micrometer with heavy lines every 
half millimeter is satisfactory. These lines may be ruled on glass 
and filled with graphite, or they may be made by photography 
(see 5o8a). 

For example, if the micrometer used has half millimeter spaces, 
the image projected on the screen will be magnified more or less 
according to the distance of the screen and the objective used. 
The exact size of the image is easily measured on the drawing sur- 
face with a millimeter scale. Suppose that two of the half milli- 
meter spaces were used as object, the object would be one milli- 
meter in actual size. If the image of two spaces projected on the 
screen or drawing surface measured 2 5 millimeters then the magni- 
fication would be 25. 



508a. One can make a satisfactory micrometer for determining the 
magnification of drawings as follows : Make a negative of the millimeter scale 
(fig. 178, 21 1) making the picture exactly half the size of the original scale, then 
the spaces will be half millimeters. As the scale is black with light lines the 
negative will show dark lines with intervening clear spaces exactly as is a glass 
micrometer. 

If so desired the micrometer lines may be covered with Canada balsam and 
a cover-glass applied as for microscopic specimens. (See The Microscope, 
354~5> P- 2 57)- This would protect the lines and make the specimen more 
transparent. A lantern-slide plate is the best for making the negative, as it 
gives transparent lines. 



354 



MAGNIFICATION OF DRAWINGS 



[Cn. X 



Briefly stated, if one has an object of known size, then the size of 
the image divided by the size of the object will give the magnifica- 
tion in every case. In the example given above the object is i mm. 
and the image is 25 mm. : 25 -=- i = 25. Or, if one took a single 
space as object, that is, half a millimeter, then the image would 
measure 12.5 mm., and 12.5 -=- .5 = 25 as before (see sioa). 




FIG. 198. SPENCER LENS COMPANY'S APPARATUS FOR DRAWIM; WITH 

THE MICROSCOPE. 
(Cut loaned by the Spencer Lens Co.). 

This consists of a small arc lamp with the proper wiring, rheostat and con- 
nections for the house electric supply. The lamp has all the adjustments, and 
the condenser tube is telescoping so that the beam of light may be parallel or 
converging. 

The microscope is supported on an adjustable shelf which can be raised or 
lowered on the vertical rods, thus enabling one to get any desired magnification. 

The vertical supports for the microscope shelf serve to carry a curved metal 
band to support the cloth curtains to shade the drawing surface. There are 
two curtains and they hang freely, thus avoiding all interference with the 
hands in drawing. If one desires, the arc lamp can be put in line with the 
microscope and the mirror turned aside. 

For a reflector beyond the ocular a prism is used, thus avoiding any of the 
defects of a mirror. 



CH. X] 



MAGNIFICATION OF DRAWINGS 



355 



MAGNIFICATION OF WALL DIAGRAMS, AND DRAWINGS MADE WITH 
THE OPAQUE LANTERN 

509. Scale of wall diagram. Make the diagram any desired 
size as directed in 457. Then remove the negative or lantern 
slide from the holder and insert a lantern slide or negative of the 
metric rule (fig. 178, 211). The image of the metric rule on the 
drawing surface will, of course, be magnified exactly as much as the 
diagram. By using a metric measure, one can find the magnifica- 
tion of the screen image exactly as described in 508. 

For wall diagrams, it is not usually very important to know the 
magnification. All that is necessary is to get it large enough to be 
seen well. But if one wishes to show the relative size of objects 
such as blood corpuscles of different animals, then the magnifica- 
tion must be known. 




\_j 

FIG. 199. MODEL 4-5 DELINEASCOPE WITH THE PROJECTION MICRO- 
SCOPE IN A HORIZONTAL POSITION. 

(Cut loaned by the Spencer Lens Co.). 

With the microscope in this position the image may be thrown down on a 
horizontal surface for drawing by pushing the totally reflecting prism up in 
the microscope tube to intercept the light (see fig. 109, 175). 



356 



MAGNIFICATION OF DRAWINGS 



[Cn. X 




FIG. 200. COMBINED DRAWING AND PHOTO-MICROGRAPHIC APPARATUS 

OF THE BAUSCH & LOME OPTICAL COMPANY FOR USE ON THE 

HOUSE LIGHTING SYSTEM. 

(Cut loaned by the Bausch & Lomb Optical Co.). 

This is a kind of universal apparatus serving for drawing with the microscope, 
projection with a microscope and with a magic lantern; opaque projection, 
and finally for photographing with all objectives and with the microscope. 
It can be used in a horizontal, an inclined or a vertical position. For 
drawing with the microscope in a horizontal position there is an adjustable 
drawing shelf with a cloth tent for shutting out daylight in a light room. 

The large condenser enables one to use the apparatus on specimens of all 
sizes up to lantern slides. 

510. Scale of diagrams or drawings made with the opaque 
lantern. If one uses the episcope or opaque lantern, or a photo- 
graphic camera for drawing, it is very easy to get the exact magnifi- 
cation of the drawing by putting a metric rule upon some part of 



CH. X] DRAWINGS FOR MODELS 357 

the object, or beside it. It will be at the same scale of magnifica- 
tion or reduction as the drawing. 

In practice some lines of the image of the scale are made beside 
the drawing. For example, suppose the image of one centimeter 
measured on the drawing was 10 centimeters long, one would know 
that the drawing is 10 times larger than the object. If the length 
of the centimeter on the drawing was only one-half centimeter long, 
then one would know that the drawing is only half as large as the 
object and so on ( 5o8a, 5ioa). 

DRAWINGS FOR MODELS 

511. Drawings for models. These are made much more 
easily with projection apparatus than with the camera lucida or in 
any other way. The simple drawing outfit for use on the house 
circuit described above makes it possible for every laboratory and 
indeed every private worker to use this effective method, even if 
complete projection apparatus and heavy lantern currents are not 
available. 

In making drawings for models several steps must be taken in 
order that the resulting model shall be anything like a true repre- 
sentation of the actual object. 

(1) The object (embryo, etc.) should be photographed at a 
known magnification before it is sectioned. 

(2) The sections should be made of a known thickness 
1 5 [A, etc.). 



510a. The general law for magnification and reduction. With a given 
object the size of the image depends directly upon the relative distance of the 
object and of the image from the center of the lens (fig. 185, 209, 210). If the 
image is farther from the center of the lens than the object then the image will 
be larger than the object; conversely if the image is nearer the center of the 
lens than the object then it will be smaller than the object. 

For example, if the image is to be ten times as large as the object the image 
must be ten times as far from the center of the lens as the object. 

Conversely, if the image is to be one-tenth as large as the object it must be 
formed only one-tenth as far from the lens as the object. 

In lantern-slide and micro-projection, and in photo-micrography the image 
is much larger than the object and correspondingly more distant from the cen- 
ter of the lens. In ordinary portrait photography and in landscape photo- 
graphy the image is much smaller than the object, and consequently the image 
is much nearer the lens than the object (see also 392a). 



DRAWING WITH THE MICROSCOPE 



ICH. X 




FIG. 201, A, B, C. SIMPLE DRAWING OUTFIT FOR THE MICROSCOPE. 

(Cuts loaned by the Bausch & Lomb Optical Co.). 



CH. XJ DRAWING FOR MODELS 359 

There is a hand-feed, right-angled arc lamp for small carbons, wiring and 
connections for the house circuit and a rheostat which will not permit over 6 
amperes of current to flow. The lamp condenser is in a telescoping tube so 
that either a parallel or a converging beam of light can be obtained. To avoid 
stray light the drawing surface is enclosed by a metal box with one side removed. 

A Drawing outfit with the lamp and microscope in line. 

The microscope is supported on a block to give a drawing distance of 254 mm. 
(10 inches). 

B Drawing outfit with the arc lamp at right angles to the microscope. 

C Drawing outfit with the microscope on an adjustable platform and the 
arc lamp at right angles with the microscope. 

(3) It must be decided in the beginning how much larger the 
model is to be than the original object. 

(4) The objective and the drawing surface must be chosen and 
mutually arranged so that the desired magnification is attained 

( 509)- 

(5) The object must be placed on the stage of the microscope so 
that the image reflected down upon the drawing surface will be 
erect, that is, exactly like the object and not inverted in any way 
(see below 512). 

(6) Each drawing as it is made must be numbered to correspond 
with the number of the section : This is very important. 

(7) It is desirable to make a duplicate set of drawings, for one 
set is used up in making the model, and one needs a set for reference. 

The duplicate drawings are easily made by using thin carbon 
paper as in duplicating writing, or in typewriting. 

(8) Marking the position of the apparatus. If all the draw- 
ings cannot be made at one time, then the objective, the ocular, 
if one is used, and the distance of the drawing surface from the 
tube of the microscope should be carefully measured or indicated 
by chalk marks, so that when working again exactly the same 
magnification can be used. It is well also to check up by using the 
stage micrometer again ( 508). Pictures for models may also be 
made by photography, (see 542). 

ERECT IMAGES 

512. It has been known from the first use of projection appar- 
atus that the projected image was inverted, and that this is true 
whether a simple aperture, a simple lens, or an objective of several 



3 6o 



ERECT IMAGES IN DRAWINGS 



[Cn. X 




The arc lamp is of the Liliput 
form with small right-angled car- 
bons. 

The lamp condenser is large, 
such as is used for lantern-slide 
projection, hence large as well 
as small objects can be illumi- 
nated by it. 

For convenience in feeding the 
carbons there is a rod extending 
down within reach of the artist. 

The microscope and stage are 
separate and independently 
movable on the vertical optical 
bench. In addition to the lamp 
condenser there are two or more 
substage condensers of different 
foci. 

The object is put on the upper 
side of the stage. 

The microscope can be used 
with an ocular, or the draw tube 
and ocular can be removed from 
the large microscope tube, and 
then objectives alone'used, thus 
givingf very large fields. 



GER'S VERTICAL DRAWING AND PHOTOGRAPHIC 
APPARATUS FOR USE ON THE 
HOUSE CIRCUIT. 

(Cut loaned by Ernst Leitz). 



CH. X] ERECT IMAGES IN DRAWINGS 361 

lenses is used. The earliest workers also saw that an easy way to 
correct for this was to invert the object, then its image would 
appear in the natural position. But some objects do not admit of 
inversion, hence the effort to obtain erect images by some optical 
means. 

The first and still the simplest method is by the use of a plane 
mirror with a horizontal screen (fig. 88, 89, 181, 204). The mirror 
might be put in the course of the beam before or after it has passed 
the objective. Figure 89 shows it before and figure 182 after 
traversing the objective. 

It was demonstrated by Kepler (1611) and practically worked 
out by Scheiner (1619) that erect images could be produced by the 
use of two objectives in line. The first objective gives a real 
inverted image of the object, and the second gives a real, erect 
image of the inverted image (fig. 208). This is what occurs when- 
ever an ocular is used with an objective in projecting with the 
microscope (fig. 207). 

The principles for getting erect images with projection apparatus 
are very simple, but in practice it is a little puzzling to decide off- 
hand just how to arrange the object so that the screen image shall 
be erect and not show any of the inversions (fig. 212-214). This 
difficulty arises from the fact that in the different kinds of projec- 
tion sometimes an opaque object is used, and sometimes a trans- 
parent object ; sometimes an opaque and sometimes a translucent 
screen is employed; sometimes an objective only, and sometimes 
both an objective and an ocular are used for projecting the image; 
and finally, sometimes it is necessary to use a mirror or prism as well 
as an objective to get the image on the vertical or horizontal surface 
where it is to be seen or drawn. 

The simplest and surest way to get the microscopic specimen on 
the stage of the projection microscope in a position which will give 
a correct image for drawing is the following: 

i. The prepared microscopic specimen is placed on a piece of 
white paper so that it appears exactly as it should in the drawing, 
and the letters a and k are written on the cover-glass between 
the specimens (fig. 220). 



362 



ERECT IMAGES IN DRAWINGS 



[Cn. X 



2 . The slide is then placed on the stage of the projection appara- 
tus and its image thrown on the drawing surface. In case the 
specimen is wrongly placed to give an erect image the letters will 
show it, and the specimen can be rearranged until the images of the 
letters are correct in every way, then of course the images of the 
microscopic specimens will be erect in every way (see also 517). 

513. Erect images with opaque objects in a photographic 
camera with translucent screen. Place the object upside down in 
the holder. On the translucent screen it will be erect (fig. 211). 
If the object cannot be put upside down, the image will appear 
wrong side up on the translucent screen (fig. 212). It can be drawn 




FIG. 203. LARGE EDINGER APPARATUS IN A HORIZONTAL POSITION FOR 

PROJECTION ON A VERTICAL SCREEN. 

(Cut loaned by Ernst Leitz). 



CH. X] 



ERECT IMAGES IN DRAWINGS 




FIG. 204. EDINGER'S OUTFIT FOR DRAWING WITH AN ORDINARY MICRO- 
SCOPE AND SMALL ARC LAMP ON THE HOUSE LIGHTING SYSTEM. 

(Cut loaned by Ernst Leitz). 

This is the first form of the drawing outfits using the ordinary microscope 
and the small arc lamp on the house lighting circuit. It was demonstrated at 
the meeting of the Anatomische Gesellschaft at its Leipzig meeting, April, 191 1. 

The microscope is inclined to 45 and the mirror at an angle of 22.5, thus 
directing the light vertically down upon the horizontal drawing surface. 

For drawing in a light room a cloth tent is provided and is supported above 
and on the sides by metal arches. If it is very light one can pull the cloth fever 
the head as in focusing a camera. In the evening or in a dark room the cloth 
can be opened widely to expose the drawing surface. 

or traced in this position and the drawing turned right side up, 
when it will appear like fig. 211, that is, correct in every way. 

514. Erect images with the opaque lantern or episcope. 

(A) The objective horizontal, the object and the drawing surface 



364 



ERECT IMAGES IN DRAWINGS 



[Cn. X 



vertical. The object is placed upside down in its vertical holder. 
The mirror reflecting the image upon the vertical drawing surface 
will give an erect image (fig. 211). 

(B) The objective and the drawing surface horizontal, the 
object vertical. The artist with his back toward the apparatus : 
Place the object right side up in the vertical holder. 

(C) Same as above, but with the artist facing the apparatus as 
with the drawing shelf in fig. 183. Place the object wrong side up 
in the vertical holder. 

(D) Same, except that a vertical translucent screen is used. 
Place the object wrong side up in the vertical holder ; do not use a 




FIG. 205. SMALL ARC LAMP WITH CLOCK-WORK FOR FEEDING THE CARBONS. 
(Cut loaned by Ernst Leitz). 

This arc lamp for the house circuit has a clock-work which moves the carbons 
continuously. The arc must be started by hand as for a hand-feed lamp, but 
when once started the lamp will burn continuously provided the carbons burn 
off as fast as they are fed. If the carbons are too large the clock-work will feed 
them together faster than they burn away, and if too small the clock-work feeds 
the carbons too slowly and the lamp will go out. 

The clock-work has a regulating device for speed and the lamp has the usual 
feed wheel for hand regulation. 

This form of feeding mechanism is equally good for direct and for alternating 
current as the movement is entirely controlled by the clock-work. Such a 
lamp is especially useful for drawing and for photography. 



CH. X] 



ERECT IMAGES IN DRAWINGS 



365 



mirror or prism with the objective, but point the objective directly 
toward the screen. 

515. Erect images of horizontal objects with the episcope. 

Vertical drawing surface and vertical objective, horizontal object. 
The object is placed with its upper edge away from the drawing 
surface and the mirror reflecting the image to the vertical screen 
will make it erect (fig. 211). 

516. Erect images on the drawing surface with the magic 
lantern. (A) With an opaque, vertical drawing surface. Place 
the transparency in the slide-carrier as described "for lantern slides 
(fig. 7-8), i. e., with the object facing the light and wrong side up. 

(B) For a translucent, vertical drawing surface. Place the 
object facing the objective and wrong side up. 

(C) For an opaque horizontal screen. Place the object so that 
it faces the objective and the mirror or prism reflecting the rays 
downward will give an erect image (see B and C in 514). 



ERECT IMAGES WITH THE PROJECTION MICROSCOPE 

517. Demonstration of the position of objects for erect 
images. The simplest way to determine how a specimen should be 
placed on the stage of the microscope to give an erect image on any 




FIG. 206. MAGIC LANTERN ARC LAMP AND TWO-LENS CONDENSER USED 
IN MICRO-PROJECTION AND FOR DRAWING. 

(See fig. 146 for full explanation). 



366 



ERECT IMAGES IN DRAWINGS 



[Cn. X 



kind or position of a screen is to use a specimen prepared as follows : 
An ordinary microscopic slide is varnished as directed for lantern- 
slide glasses (Ch. VIII, 317) and then the small letters a and k are 
written in the middle with a fine pen. These letters are selected 
because both in script and in printing they indicate clearly which 
side up they are and which way they face. With some letters it is 
not so easy to determine whether they have suffered an inversion 
or not. 

A low power, 50 to 100 mm. focus objective, is good for projecting 
the image. 

One could use a lantern slide with print upon it, or even a nega- 
tive. For our experiments we used a lantern slide or negative of 
the metric measure (fig. 178, 211) so that cuts could be made for 
this book which were exactly like the images obtained on the screen 
with the transparency in different positions. 



Ocular 



Objective 




FIG. 207. DIAGRAM OF THE COURSE OF THE RAYS AND THE POSITION OF 
THE IMAGES WHEN AN OCULAR is USED. 

Object The object whose image is to be projected. 

Objective The projection objective. 

// Field lens of the ocular. It acts with the objective to give a real, 
inverted image r i. 

r i The real, inverted image of the object formed by the objective and the 
field lens of the ocular. 

r 1 *'* The inverted image of the object which would be formed by the objec- 
tive if the ocular were absent. 

e I Eye lens of the ocular. It acts like a second projection objective and 
gives a screen image of the real image (r i). 

Axis The optic axis of all the lenses. 

Screen Image The image projected by the eye lens. This image will be 
right side up, but the rights and lefts will be reversed on the ordinary opaque 
screen. If a translucent screen is used and the observer is behind it, the image 
will appear erect, and the rights and lefts will not be reversed. 



CH. X] ERECT IMAGES IN DRAWINGS 367 

It is a good plan to have a specially prepared microscopic slide or 
a lantern slide with print at hand whenever one is going to draw, 
then one can determine quickly and exactly how the specimen 
should be placed to give an erect image. A simpler method still 
is to write the letters, a, k, on the cover of the specimen to be 
drawn (512, fig. 220). 

POSITION OF THE OBJECT FOR ERECT IMAGES WITH THE PROJECTION 

MICROSCOPE AND AN OBJECTIVE ONLY, OR WITH AN OBJECTIVE 

AND AN AMPLIFIER 

518. For an opaque vertical screen. Place the object on the 
stage as a lantern slide is placed in its carrier (35), that is, with the 
specimen facing the light and the lower edge up. With a micro- 
scopic specimen this would bring the cover-glass next the stage and 
facing away from the objective instead of toward it, as in ordinary 
microscopic observation. In this case one must focus through the 
slide instead of through the cover-glass. This can, of course, be 
done with low, but not with high powers. (See drawing on a hori- 
zontal surface 524). 

With the specimen placed as directed, the image on the vertical 
opaque screen will appear erect in every way (fig. 211). 

If one faces the light and looks at the specimen on the stage it will 
look like fig. 214 that is, like print seen through a sheet of paper 
wrong side up. 

519. For a translucent vertical screen. If the screen is of 
ground-glass like that of a photographic camera, or if it is of tracing 
paper or other translucent substance supported by clear glass, the 
object should be placed on the stage so that it faces the objective, 
and is lower edge up. 

When the observer looks at the image on the translucent screen, 
i. e., facing the light, the image will be erect like fig. 211. 

When he faces the light and looks at the object on the stage it will 
appear like fig. 212, i. e., it is simply upside down. 



368 



ERECT IMAGES IN DRAWINGS 



[CH. X 



POSITION OF THE OBJECT FOR ERECT IMAGES ON A HORIZONTAL 

SURFACE WITH AN OBJECTIVE OR WITH AN OBJECTIVE AND AN 

AMPLIFIER AND A 45 DEGREE MIRROR OR PRISM 

520. For an opaque horizontal screen. (A) If for a drawing 
table and mirror (fig. 182), place the object on the stage so that 
it faces the objective and is right edge up. The image on the 
horizontal surface will appear erect when the observer looks at it 
facing away from the light. 

The object on the stage will appear erect when the observer looks 
at it facing toward the light. 




Fip 208. KEPLER'S METHOD OF PRODUCING ERECT IMAGES BY MEANS 
OF Two PROJECTION LENSES. 

(From Scheiner's "Oculus" , 1619). 



CH. X] ERECT IMAGES IN DRAWINGS 369 

(B) If the mirror is very close to the objective (fig. 183) the 
natural position for drawing is to sit facing the light. The object 
then is put in position facing the objective as before, but upside 
down. The image will appear erect on the drawing surface when 
the observer faces the light. 

521. For a translucent, horizontal screen. In some of the 
old forms of sketching apparatus the image was reflected upward 
by a mirror or prism, and the artist drew on the upper surface. 




FIG. 209. DIAGRAM TO SHOW THAT THE SIZE OF THE IMAGE OF AN OBJECT 

DEPENDS UPON THE RELATIVE DISTANCE OF THE OBJECT AND IMAGE 

FROM THE CENTER OF THE PROJECTION LENS. 

(From. The Microscope}. 

In this figure the image is four times as far from the center of the lens (cl) 
as the object, hence, from the law of similar triangles, the image must be four 
times as long as the object. 

For such an arrangement, the object is put on the stage facing the 
light, but right edge up. The image will appear erect on the 
translucent screen when the observer faces the light and looks down 
upon the screen. For this experiment the mirror or prism must be 
on the lower side of the ocular (fig. 215). 

POSITION OF THE OBJECT FOR AN ERECT IMAGE WITH AN 
OBJECTIVE AND AN OCULAR 

522. For an opaque vertical screen. The object should face 
the light as with a lantern slide, but it must be right edge up. 
With a microscopic specimen the cover-glass will be next the stage 
as in 518. On the screen the image will appear erect (fig. 211). 
The object on the stage will appear reversed like print seen in a 
mirror (fig. 213). 



37 



ERECT IMAGES IN DRAWINGS 



[CH. X 




Objegtive 



Objecl-h 
Object-a 



FIG. 210. DIAGRAM TO SHOW THAT THE SIZE OF THE. IMAGE DEPENDS 

UPON THE DISTANCE OF THE OBJECT FROM THE CENTER OF THE LENS. 

(From The Microscope). 

The object at Object-a necessitates an image at Image-a; while if the same 
object is moved closer to the lens, as at Object-b, the image will move farther 
from the lens (Image-b) and be correspondingly larger. 

// The principal foci of the lens (objective). 

axis The principal axis of the lens. 

Secondary axis a, Secondary axis b Represent the secondary axes which 
mark the limit of the object and the two images. 

With the object farther from the lens the secondary axes are in full lines, 
while for the object nearer the lens the secondary axes and the image are shown 
by broken lines. 

523. For a translucent vertical screen. The object is put on 
the stage facing the objective and right edge up. The image will 



CH X] ERECT IMAGES IN DRAWINGS 

1O CENTIMETER RULE 




The upper edge is in millimeters, the lower in centimeters 

FIG. 211. CORRECT IMAGE. 
\ 

ni J3/v\o[ sqi 'saapminim ui si sSpa jaddn 




H3X3JMIXN3O Ot 

FIG. 212. INVERTED IMAGE. 



3JEUH 




ni iswof sd) .aiaJamillim ni zi 9-gbs isqqu 9iiT 

FIG. 213. MIRROR IMAGE. 
nbbei. cqSc K in njijjiroG^Gi.3' (p& JOMGX in CGnfiniGfGi.3 1 




TO CEMJJIALEXEB 

FIG. 214. INVERTED MIRROR IMAGE. 

FIG. 211-214. FIGURES OF A METRIC RULE, FULL SIZE, TO SHOW CORRECT, 
INVERTED, MIRROR AND INVERTED MIRROR IMAGES. 

These representations of screen images show the result of placing the object 
in different positions or of using different means in projection. The determin- 
ing factors for the position of the object for a correct screen image are: 

(1) Projection by an objective or by an objective and an amplifier (fig. 121, 
126). 

(2) Projection by means of two lenses or of an objective and an ocular 
(fig. 207, 208). 



372 



ERECT IMAGES IN DRAWINGS 



ICn. X 



(3) The use of a prism or of a mirror to change the direction of the rays on 
their way to the screen (fig. 192). 

(4) The use of an opaque screen. 

(5) The use of a translucent screen. 

appear erect like fig. 211 when seen through the translucent screen 
and facing the light. 

Facing the light, the object on the stage will also appear erect. 



POSITION OF THE OBJECT FOR AN ERECT IMAGE WITH AN OBJECTIVE 

AND OCULAR, AND A 45 DEGREE MIRROR OR A TOTALLY 

REFLECTING PRISM 

524. For an opaque horizontal screen. (A) For the draw- 
ing table and mirror (fig. 182), place the object on the stage so that 
it faces the objective and is with the lower edge up. The image will 
appear erect on the drawing surface when the observer faces away 
from the light. 




FIG. 215. EARLY METHODS OF DRAWING WITH PROJECTION APPARATUS. 

In the picture at the left (Fig. 6) is shown a drawing tent or box with a 45 
mirror and vertical objective by which an erect image is projected upon the 
drawing table as in figures 88-89. The artist sits outside, but has his head 
and bust within and the light is excluded by a cloth curtain over the back. 

In Fig. 5 is shown a drawing box composed of an objective at the right (CD), 
a 45 mirror (E F), and a drawing surface (C) covered by a sloping roof of 
opaque material to keep out the light. With this instrument the artist simply 
introduces the hand and pencil. The picture will have the rights and lefts 
reversed as the drawing is made on the back of the drawing paper, not on the 
front as with Fig. 6. 

Fig. 4 is to show the course of the rays from an object (A B), and its inverted 
image (G H). When the mirror (E F) is introduced the image (/ K) is rendered 
horizontal. 



CH. X] DRAWING FOR PUBLICATION 373 

If the observer faces the light the object on the stage will appear 
like a printed page upside down (fig. 212). 

(B) For a drawing shelf, the mirror or prism being close to the 
ocular and the draughtsman sitting with his face toward the light 
(fig. 183, 187), the object is placed on the stage facing the objective 
and right edge up. 

The image will be erect on the drawing surface (fig. 211). 

The object on the stage will also appear erect (fig. 211). 

525. For a translucent screen. For this the object is simply 
turned around so that it faces in the opposite direction in each case 
but remains the same edge up. 

526. For erect images on a horizontal drawing surface with 
apparatus like Edinger's (fig. 202). In this case no mirror or 
prism is necessary. The position of the object on the stage for 
erect images is precisely the same as for a horizontal microscope 
and a vertical screen ( 518). 

This has the disadvantage of requiring one to turn the cover- 
glass away from the objective, which prohibits the use of high 
powers. If the cover-glass is turned toward the objective the 
drawing will be like a mirror image (fig. 213). 

DRAWINGS FOR PUBLICATION BY THE AID OF PROJECTION 
APPARATUS 

527. Projection apparatus can give much assistance in pro- 
ducing the outlines and main details of drawings for publication. 
The outline drawings should be made on good drawing paper with a 
medium lead pencil. When the ink, air-brush, or crayon work is 
added for the finished drawing, the pencil lines will be covered or 
they may be erased. The finishing must be done free-hand and 
constant reference made to the actual specimen, to the image on 
the screen, or as looked at through a microscope. The finishing 
cannot be done successfully with the image of the specimen pro- 
jected on the drawing paper as one cannot tell how the drawing 
looks with the image projected upon it. By means of a suitable 
screen the image may be cut off of part of the drawing surface while 
doing the finishing. By removing the screen the image can be 



374 DRAWING FOR PUBLICATION [Cn. X 

projected again upon the surface to make sure that all the details 
have been correctly drawn. 

It is always desirable that drawings accompanying a scientific 
article should be at a definite enlargement or reduction, and that 
the scale of the drawing be definitely stated (See Style Brief, of the 
Wistar Institute, pp. 16-17). 

If the drawings have been made without first doing this, then 
the magnification can be found by arranging the apparatus exactly 
as when the drawings were made and using a micrometer as directed 
in 508. 

A plan frequently followed is to have a few lines of the microme- 
ter image drawn in one corner near the picture. Then any one 
can determine the scale of magnification or reduction ( 510, sioa). 

528, Lettering the drawings. After the drawings are finished 
the various parts can be lettered, or words can be written in where 
needed. Most workers, however, cannot letter neatly enough for 
publication. For such it is better to use printed words, letters or 
numerals. 

It is assumed here that the drawings will be reproduced by some 
photo-engraving process ; and for this the letters or words pasted 
on the drawing would best be printed on tissue paper, ( 528a); 
Gothic type is best. By consulting fig. 216, one can select the 
proper size for the reduction to be made ( 531). 



527a. Tracing pictures natural size on drawing paper. It frequently 
happens in making the drawings for a book or for a scientific paper that pic- 
tures from other books or scientific papers are desired. Of course, if there are 
to be no modifications, the simplest method is to borrow an electrotype or to 
have the photo-engraver make a new cut; but sometimes only an outline is 
needed or modifications are desired. 

If the picture is to be the same size as the one in the book or periodical it 
can be easily traced upon the drawing paper as follows: In place of a wooden 
shelf on the table (fig. 183, 460) a piece of thick glass is placed on the brackets 
and an incandescent lamp of 40 or 60 watts, surrounded by a lamp shade of 
some kind, is turned so that it shines directly upward. It is then placed up 
close to the glass and the picture to be traced is placed on the glass, and over 
it the drawing paper. The light is so strong that it traverses the picture and 
the drawing paper and the picture is clearly seen on the upper side of the 
drawing paper. It can be traced almost as easily as if an image were projected 
upon the upper face. In tracing drawings for this book, Wattman's hot pressed 
paper and Reynold's bristolboard were used in making tracings in the way just 
described. Even if there is print on the opposite side of the page the tracing 
of the picture can be made successfully. 



CH. X] DRAWING FOR PUBLICATION 375 

529. Fastening the letters to the drawing. The letters, 
numerals, or words are cut from the printed sheet, with pains to 
make straight edges and square corners. Then they are turned 
face downward and with a camel's hair brush of small size such 
as is used by artists, some freshly made starch paste is put on the 
back. As each word or letter is pasted, one uses fine forceps to 
pick it up and place it in the desired position, being sure that the 
letter or word is arranged properly. In the beginning it is well to 
use a try-square or some other instrument to make sure that the 
letter or word is arranged correctly. Then it is pressed down, using 
some tissue paper over the finger or some fine blotting paper, and 
pressing directly downward so as not to disarrange the letter or 
word by a lateral thrust. 

530. White letters on a black back-ground. Sometimes it is 
necessary to use white letters or numerals on a black ground (e. g., 
see fig. 211-214). In the largest printing houses one might be 
able to get these, but they are easily made as follows : 

The desired letters, numerals, abbreviations or words are printed 
on the white tissue paper as indicated above. A sheet of this 
printed tissue paper is used as a negative by putting a clean glass 
in the printing frame, placing the printed tissue paper face down 
on the glass, and then putting some Velox, Cyco, or other photo- 
graphic paper in place and printing exactly as for any negative. 
The opaque letters will be in white, and the practically transparent 
tissue paper between the letters will give the black back-ground in 
the print. 



528a. (i) The authors are indebted to Mr. George C. Stanley, Ithaca's 
photo-engraver, for the suggestion to use tissue paper for the printed letter s 
and words to be pasted on drawings for photo-engraving. The advantage o f 
tissue paper is that there is no shadow around the edge of the letter or word . 
Where thick, ordinary white paper is used there is frequently left a black line 
due to the shadow, and this line must be cut out by the engraver or it will give 
a black line in the printed book. 

(2) Starch paste for use in sticking on the letters and words should be 
freshly made. A good paste is made of dry laundry starch 5 grams, cold water 
50 cc. These are put in a small vessel and gradually heated with constant 
stirring until the paste is formed. Mucilage and other adhesives make the 
tissue paper yellowish ; and paste which has been made some time is liable to 
have fine lumps in it so that the letters are torn or distorted in pressing them 
down on the drawing. 



376 DRAWING FOR PUBLICATION [Cn. X 

Paper and developer should be of the contrast variety to give 
pure blacks and whites. 

These letters, etc., are cut out and pasted on the drawing just as 
described above. The photographic paper being rather thick, 
there will be a white streak around the letter, etc., where cut out. 
This can easily be blackened after being stuck in place by the use 
of a pen or a fine brush, using India ink. 

SIZE OF DRAWINGS AND THEIR LETTERING 

531. It is wise to make the drawings considerably larger than 
the desired picture. In reducing, the coarseness and some other 
infelicities of the drawing become less noticeable. 

Of course if the drawing is made exactly the size of the desired 
cut, then it must look exactly as one desires it in the printed book ; 
it is not liable to be improved by the process of photo-engraving. 
But if the drawing is to be reduced, then the lettering, etc., must 
be coarse enough in the drawing to give the proper appearance in 
the finished cut. 

There is some confusion in the minds of the inexperienced as to 
the appearance of a picture half the size of the original. To the 
engraver half-size always means that any given line or part is just 
half the length of the original. That is, if any line of the original 
were 10 centimeters long, the finished cut would show the same 
line 5 centimeters long if it were reduced to half the original size. 
The appearance is well shown in the accompanying figure (fig. 216). 
In the upper half the letters and numerals are of full size ; in the 
middle they are of half the original size; and below they are of 
one-fourth the original size. This picture will show one also about 
the size of type to use for the different reductions. The numerals 
on the left indicate the size of the type, as 24 point, 18 point, 12, 
10, 8, and 6 point, respectively. 

The lettering of pictures in books and periodicals should be 
proportioned to the size of the details of the cuts. It is distressing 
to have the letters and numerals on figures the most prominent 
feature. On the other hand, it is exasperating to have letters 
so minute that one needs a microscope to make them out. As 



24 Point Type A a 
123456789 10 

18 Point Type ARS 2 34 

12 Point Type ABCabc 1234 
10 Point Type ABC abc 12345 

8 Point Type ABCD abed 12345 
6 Point Type ABCDabcd12345 I II III IV 
ABCD abod 123456789 10 I II til IV V VI 



24 Point Type A a 
123456789 10 

18 Point Type A R S234 

12 Point Type ABCabc 1234 
10 Point Type ABC abc 12345 

8 Point Type ABCO aocd 12345 



4 



24 Point Type A a 
123456789 10 

18 Point Type A R S 2 3 4 



FIG. 216 



378 PHOTOGRAPHIC ENLARGEMENTS [CH. X 

FIG. 216. GOTHIC TYPE TO USE ON DRAWINGS AND THE APPEARANCE 
WHEN REDUCED. 

In the upper half are shown letters and figures of full size with their designa- 
tions by the printer, i. e., 24, 18, 12, 10, 8 and 6 point type. 

In the lower half are shown the same reduced to one-half the length, and 
reduced to one-fourth the length. 

shown by the numerals and letters in fig. 216, if the drawing is 
not to be reduced at all one can use 6, 8, or possibly 10 point type. 
For one-half reduction (one-half off), the lettering should be 
with 10 or 12 point type. For one-fourth size (^ off), then the 
lettering should be with 12 or preferably with 18 point type. 

PROJECTION APPARATUS FOR PHOTOGRAPHIC ENLARGEMENTS 

532. Enlarged prints of negatives. There is great advantage 
in making pictures of large objects at a considerable distance so that 
the perspective will be correct and all levels in focus. It is also 
advantageous to make pictures of microscopic objects without 
undue enlargement, then there is greater sharpness of the object 
as a whole. 

If now one wishes a large picture, any good negative can be 
printed by the aid of a photographic objective at almost any 
desired enlargement. This can be done with projection apparatus 
in a dark room by the following method : The management of the 
projection apparatus is as for drawing. The negative is placed in 
some kind of a holder and put in the cone of light from the main 
condenser where the part to be enlarged will be fully illuminated 
(fig. 132, 185). Care must be taken to so place the negative that 
an erect image will appear on the printing paper ( 512). 

533. Condenser required with negatives of different sizes. 

Remember that the diameter of the condenser must be somewhat 
greater than the diagonal of the part of the negative to be enlarged 
(314 and fig. 114). For example, to use the whole of a lantern- 
slide negative (85 x 100 mm., 3^ x 4 in.) the condenser should 
have a diameter of 14 cm. (5^ in.). 

For a negative 100 x 125 mm. (4x5 in.), the condenser should 
be 1 8 cm. (7 in.) in diameter; fora negative 125 x 175 mm. (5x7 in.), 
the condenser should be 23 cm. (9 in.) in diameter and for a nega- 
tive 200 x 250 mm. (8 x 10 in.), the condenser should be 35 centi- 



CH. X] PHOTOGRAPHIC ENLARGEMENTS 379 

meters (14 in.) in diameter. Of course, if only a part of the nega- 
tive plate contains the picture to be enlarged then a smaller con- 
denser in the given case will answer. The above figures are for 
the diagonal of the respective sizes. These condensers are usually 
of relatively long focus, especially those of the larger diameters, 
e. g., the 35 cm. lens ordinarily has a focus of 50 centimeters. The 
condensers most used for enlarging are usually of the double form, 
the convexities facing each other as for the magic lantern condenser 
(%. 185). 

534. Objectives to use for printing. It is necessary to use 
an objective which has been corrected for photography. The 
ordinary projection objective gives a good visual image, but not a 
good photographic image, hence it is better to use a photographic 
objective. 

535. In focusing, some white paper should be put into the 
printing frame or pinned in place and the image focused with care. 
The photographic paper when put in the same place will then give 
a sharp picture. 

536. Photographic paper for printing with projection appara- 
tus. If one has sunlight or the arc light the developing papers like 
Velox, Cyco, etc., are plenty rapid enough. If a weak light is all 
that is available, then Haloid or one of the more rapid bromide 
papers will be called for. 

537. Holding the paper while printing. (A) If the pictures 
are of microscopic objects or other pictures of relatively small size 
(i. e., up to 30 x 35 cm.; 12 x 14 in.), a good method is to put a 
clear glass in a printing frame and press the printing paper down 
upon it just as one does for printing from a glass negative by con- 
tact. This holds the paper perfectly flat and ensures uniform 
sharpness. With the printing frame one can lay it flat if a mirror 
or prism is used, or it can stand on edge facing the objective if no 
mirror is used. 

(B) If the printing paper is large the usual method is to have a 
board screen on a track. The picture is then got of the desired 
size by varying the distance between the board and the objective, 



3 So PHOTOGRAPHIC ENLARGEMENTS [Cn. X 

then the image is carefully focused by putting some white paper 
on the screen or by having a ground-glass in the middle of the 
screen. Then the objective is covered with a dark cap or with a 
cap containing ruby glass, and the photographic paper is fastened 
in place by thumb tacks or in some other way, care being taken to 
stretch it smooth. 

538. Exposure. When the paper is in place the cap is 
removed from the objective and the projected image will print on 
the paper. The time necessary depends upon the magnification, 
the density of the negative, the intensity of the light and the sensi- 
tiveness of the paper used . It usually takes about one-fourth the time 
one would print by contact using a 1 6 candle-power frosted incan- 
descent lamp. A good plan is to try a small piece of the paper and 
determine the correct exposure before printing on the large sheet. 
After the exposure the objective is covered with the cap and the 
paper is developed exactly as for contact printing. 

539. Diaphragm of the objective. In printing, the diaphragm 
of the objective is wide open if the unmodified cone of light is used 
for illumination. This has one defect with the arc lamp. If there 
are any irregularities in the negative, such as minute scratches, etc., 
they would show in the print, whereas if the illumination were from 
an extended instead of a very small source like the crater of the arc 
lamp, the slight defects would show very much less. 

To obviate this defect with the arc lamp one or more plates of 
ground-glass or of milk white glass are placed in the path of the 
beam before the negative. It must be put far enough from the 
negative so that the grain of the ground-glass will not show. 

With the ground-glass or the milky glass in the beam the dia- 
phragm of the objective can be closed as much as desired. The 
use of the ground-glass and the closure of the diaphragm will, of 
course, necessitate a longer exposure. 

540. Avoidance of stray light. If one is to do considerable 
printing with the projection apparatus a light-tight lamp-house 
must be used and light-tight bellows between the condenser and 
the negative and objective. A special camera is most satisfactory. 



CH. X] PHOTOGRAPHY AND PROJECTION 381 

For the occasional use of a laboratory the stray light can be ex- 
cluded by means of asbestos paper. Sometimes the arc lamp is 
put on the outside of a partition, but that necessitates going out of 
the printing room to adjust the lamp. If direct current is available 
an automatic lamp is a great convenience. 

PHOTOGRAPHING WITH PROJECTION APPARATUS 

541. Apparatus which will give good projection of micro- 
scopic specimens can, with slight modifications be used for photo- 
micrography. 

There are three possibilities: 

(1) Printing the image directly on a developing paper. 

(2) Exposing a dry plate directly to the image as for the paper. 

(3) Using a camera and plate holder. 

542. Printing the projected image directly on a developing 
paper. With the apparatus set up exactly as for drawing one can 
expose a sheet of developing paper to the sharply focused image of 
the specimen as described for the enlargement of negatives ( 532). 
The lights and shades will be reversed, but all the outlines and 
details will be present. This is a convenient method of getting an 
enlarged record of the specimen. 

It is also a good method for making pictures for models ( 511) 
especially when there are many details. With the cheap develop- 
ing papers in rolls now obtainable the expense is not greater than 
for making drawings, and there is liable to be a gain in accuracy. 
The main draw-back is that but a single picture is made of each 
specimen for a single exposure, while in drawing it is as easy to 
make two or three as one, by using carbon paper ( 511). 

543. Exposing a dry plate directly to the image. A dry plate 
may be exposed as just described for the developing paper. The 
object must be so placed on the stage of the microscope that the 
image on the screen will be a mirror image of the specimen, that is, 
the rights and lefts will be reversed as they should be in a negative. 
The image is sharply focused, and the light cut off with a deep red 
glass so that the plate will not be affected. 



382 



PHOTOGRAPHY AND PROJECTION 



[CM. X 



A Set screw holding the rod (5) in any de- 
sired position. 

P Q Set screws by which the bellows are 
held in place. 

B Stand with tripod base in which the sup- 
porting rod (S) is held. This rod is now grad- 
uated in centimeters and is a ready means of de- 
termining the length of the camera. 

M Mirror of the microscope. 

L The sleeve serving to make a light-tight 
connection between the camera and microscope. 

O The lower end of the camera. 

R The upper end of the camera where the 
focusing screen and plate holder are situated. 



The plate holder is then put in po- 
sition, and the dark slide removed. 
The red glass is then removed for the 
short time necessary for the exposure 
O/i oth sec., more or less) and then re- 
placed. The dark slide is put back in 
the holder. The plate is developed and 
printed as usual. 

When working with dry plates in this 
way great care is required to avoid stray 
light. Stray light which would not in- 
jure printing papers will fog a dry plate. 

544. Using a camera and plate 
holder. When exact results are required 
or much photo-micrography is to be undertaken, it is better to use 
a camera in connection with the projection apparatus (fig. 219). 

The camera and projection apparatus are put on a long labora- 
tory table, or the camera may be put on a second table and adjusted 
to the height of the projection microscope. The camera is con- 
nected with the projection microscope by means of a light-excluding 
sleeve such as that used by Zeiss with his photo-micrographic 
outfit (fig. 217-218). 

The camera serves to exclude all stray light and to hold the 
plate holder in the correct position. The camera is supplied with a 
focusing screen which occupies exactly the same position as doe& 
the plate during exposure. 




FIG. 217. VERTICAL PHOTO- 
MICROGRAPHIC CAMERA. 
(From Zeiss' Photo-micro- 
graphic Catalogue). 



CH. Xj 



PHOTOGRAPHY AND PROJECTION 



383 




FIG. 2I7A. VERTICAL CAMERA WITH METAL SHIELD. 
(From the Transactions of the Amer. Micr., Soc., Vol. XXIII, 1901). 

The camera is on a low table and a shield of sheet zinc or roofing tin is on a 
stand between the source of light and the camera to protect the camera and 
the eyes of the operator. Opposite the light source is an opening with a shutter 
The source of light in this case is a kerosene lamp. 



384 PHOTOGRAPHY AND PROJECTION [Cn. X 

545. Objectives to use. The microplanars (fig. 123) or 
other short focus objectives of the photographic type are used 
without an ocular. They can be screwed into the nose-piece of the 
microscope or the microscope can be dispensed with and the 
objectives put into the end of the camera as with photographic 
objectives. 

If one wishes to use the ordinary microscope objectives then an 
ocular of the projection type is of great advantage although 
Huygenian oculars will give good results. The apochromatic 
objectives, and the projection or compensation oculars to go with 
them, give the best results. 

546. Making the negative. The image is first focused very 
sharply on the focusing screen. For lights of high intensity it will 
be necessary to soften the light in focusing so as not to injure the 
eyes. This can be done by putting a neutral tint glass plate or 
several thicknesses of ground -glass or one or more plates of milky 
glass in the path of the beam before the object. 

The exposure and subsequent development and printing with the 
negative are as usual. 

547. Photo-micrography with a vertical camera. If a ver- 
tical microscope is to be employed for photography, then it is best 
to use a vertical camera (fig. 217). A parallel beam of light is 
caused to fall upon the plane surface of the microscope mirror, and 
the mirror is turned to throw it directly up through the substage 
condenser upon the object. To get the parallel or approximately 
parallel beam one uses a condenser lens of very long focus ( 479, 
fig. 154) or a parallelizing lens is used (fig. 153). 

TROUBLES MET IN CHAPTER X 

548. The troubles liable to arise in the work of this chapter 
are those common to the preceding chapters. Those discussed in 
Chapter I and III are to be especially reviewed, as the source of 
light is most likely to be the electric arc. (See i28a for the 
blowing of fuses). 

(i) In drawing with the microscope with the small carbon arc 
lamp on the house lighting system, probably the trouble most 



CH. X] 



TROUBLES IN DRAWING 



385 




FIG. 218. THE ZEISS PHOTO-MICROGRAPHIC MICROSCOPE. 

(From Zeiss' Catalogue). 

This is the parent form of photo-micrographic stands with large tube (J 1 ), 
handle in the pillar and a special fine adjustment at the side (W). At the top 
is half of the light-excluding sleeve. 



TROUBLES IN DRAWING [Cn. X 

likely to arise is the lack of a brilliant picture on the drawing paper 
owing to the light in the room. Remember that to get a brilliant 
image the light must come to the eyes from the drawing surface 
only, and the drawing surface must receive no light except that 
from the specimen. The weaker the light and the greater the 
magnification the darker must the room be. 

(2) In drawing from negatives or lantern slides remember that 
it is necessary to have a condenser somewhat larger than the 
diagonal of the object to be drawn ( 314, 533). 

(3) In drawing with the microscope where the substage con- 
denser is used the condenser must be in the exact position to give 
the best results. If the slide is thick the condenser is a little higher 




FIG. 219. MICRO-PROJECTION OUTFIT AND VERTICAL CAMERA ARRANGED 

FOR PHOTO-MICROGRAPHY. 

(From The Microscope). 

The apparatus is set up on a long table or on two tables placed end to end. 
The vertical camera (fig. 217) is placed horizontally and the bellows reversed. 
For illumination a petroleum lamp with large flat wick (38 mm., \y 2 in.) 
answers well. 

Objects 50 to 60 mm. in diameter may be fully illuminated with the face of 
the flame, the lamp being i to 2 centimeters from the condenser. For powers 
of 100 to 150 diameters the flame is turned obliquely or edgewise, and placed 
5 to 6 centimeters from the condenser. The position shown in the picture 
above is for high power work. No water-cell or specimen cooler is needed. 

A light-tight connection is made with the large tube of the microscope by a 
double sleeve like that employed by Zeiss for the microscope. With low 
magnifications no ocular is used, and the objective is placed in the end of the 
camera. If one desires to make pictures of a size above the capacity of the 
photo-micrographic camera it is possible to use an ordinary camera, (fig. 117- 
119), then even quite large objects 50 to 60 mm. long, can be magnified con- 
siderably. The petroleum lamp has some advantages over daylight as the 
lamp gives an illumination of constant intensity. It is available during the 
entire 24 hours of the day, and in all seasons. 



CH. X] 



TROUBLES IN DRAWING 



387 



than for a slide which is thin. Attention to the substage con- 
denser will make a great difference with one's success. 

(4) The right-angled arc lamp should be used in drawing 
because if the microscope and lamp are properly arranged the source 
of light will remain in the axis no matter how long the lamp burns. 
If an inclined carbon lamp or one with both carbons in the vertical 
or horizontal position is used the source of light is constantly 
getting out of the axis from the burning away of the carbons, 
consequently they must be fed up more frequently to keep the 
source of light in the field. 

(5) The picture will be distorted unless the axial ray strikes the 
drawing surface at right angles. Therefore, in using a prism or 
mirror for a horizontal surface the microscope must be horizontal 
and the mirror or prism at 45 degrees to reflect the axial ray ver- 
tically downward. If the mirror or prism is twisted over to one 
side the axial ray will not strike the surface at right angles and there 
will be distortion. If one has a micrometer in squares it is easy to 
determine whether the image is distorted or not. 

(6) The image will be erect only when the object is properly 
placed on the stage. 

(7) If a glass mirror silvered on the back is used, and the object 
is quite opaque the secondary image reflected from the face is 




FIG. 220. SLIDE OF SERIAL SECTIONS WITH -a, k- ON THE COVER-GLASS 

TO ENABLE ONE TO DETERMINE WHEN THE IMAGE ON THE DRAWING 

SURFACE is ERECT (See fig. 143, and 512, 517). 



388 



DO AND DO NOT IN DRAWING 



[CH. X 



liable to cause confusion. If the mirror is silvered on the face or 
if a prism is used there will be no doubling of the reflected image. 

(8) Inverted images. One must follow carefully the directions 
or there is liable to be an inversion of the projected image ( 512- 
526). 

(9) In printing and photographing with projection apparatus 
the difficulties likely to be met with in photography are sure to come 
in. Knowledge of the principles underlying photographic pro- 
cesses will help one to overcome the troubles. 



549. Summary of Chapter X : 
Do 



i . Have a suitable room or a 
suitable shield around the draw- 
ing to keep out stray light 
( 453-455)- 



Do NOT 



i. Do not try to draw with 
the drawing .surface flooded with 
stray light. Only the light 
from the specimen should reach 
the drawing surface. 



2. Draw in the evening if a 
proper room is not available in 
the day time (453)- 



3. Use an arc lamp for light 
if possible ( 461-462, 486-487). 

4. Always use a rheostat with 
an arc lamp, large or small 
(487,%. 182,185). 

5. One can draw images pro- 
jected by all forms of projection 
apparatus (452). 



2. Do not forget that it is 
dark in all rooms in the evening 
and, therefore, that is a good 
time to draw. 

3. Do not use a weak light 
for drawing if you can have an 
arc light. 

4. Never try to use an arc 
lamp, large or small, without a 
rheostat in series with it. 

5. Do not forget that it is 
possible to draw the images 
projected by any form of appar- 
atus. 



CH. X] 



DO AND DO NOT IN DRAWING 



389 



6. In drawing with any form 
of projection apparatus the 
axial ray must strike the draw- 
ing surface at right angles or 
the image will be distorted 
(483)- 

7. Make sure that the mirror 
or prism reflects the rays upon 
the drawing surface so that the 
axial ray is at right angles to 
the surface (482-483). 

8. Use a condenser of suffi- 
cient diameter to fully light the 
object ( 467, 533). 

9. Get the desired size for the 
drawing by making the distance 
of the drawing surface greater 
or less, or by using a different 
optical system for the projec- 
tion (465, 507-508). 

10. Take great pains to light 
as brilliantly as possible ( 497, 
and Chapters I, II, and IX). 

11. Take care to have the 
images on the drawing surface 
erect ( 512-526). 

12. In using projection ap- 
paratus for photography, re- 
member the principles of good 
projection, and the require- 
ments for good photography. 



6. Do not draw distorted 
pictures; therefore do not have 
the axial ray strike the drawing 
surface obliquely. 



7. Do not forget to incline 
the mirror used in drawing so 
that the axial ray will strike the 
drawing surface at right angles. 

8. Do not try to project an 
object larger than the diameter 
of the condenser lenses used. 

9. Do not neglect to give the 
scale at which every drawing is 
made. 



10. Do not expect good light 
unless the conditions for it are 
met. 

11. Do not draw inverted 
images. 

12. Do not expect projection 
apparatus to give good photo- 
graphs unless sharp, brilliant 
images are projected, and 
the photographic part is done 
correctly. 



CHAPTER XI 
MOVING PICTURES 

550. Apparatus and Material for Chapter XI : 

A competent operator ( S5oa); Moving picture head, or mech- 
anism; Rheostat for direct current, or rheostat, inductor or 
choke-coil, transformer, rectifier, motor-generator set for alter- 
nating current ; Arc lamp and lamp-house ; Condenser, assortment 
of different sized plano-convex lenses 14, 15, 16, 17, 19, 21, 23 cm. 
focus ($yf, 6, 6^2, 7, 7>, 8, 9 in. focus); meniscus lens, 23 cm. 
focus (9 in. focus); Projection objective, equivalent focus n to 
15 cm. (4^2 to 6 in.), preferably of 6.3 cm. (2^2 in.) diameter, 
although 4.5 cm. (i^ in.) will answer; Moving picture films ; 
Tools, asbestos gloves, pliers, screw driver, copper wire, pins, 
film cement; Supply of carbons. 

For continuous use a special operating room separated from 
the auditorium by fireproof walls, all openings into the auditorium 
to have automatic shutters closing in case of fire, the room to be 
provided with a large flue connecting to the outside of the 
building. 

SSOa. Competent operator. As no one can learn a difficult art from book 
directions alone without spending an undue amount of time, we strongly advise 
every one who wishes to be a moving picture operator or photographer to get 
the help of an expert. Every university and technical school worthy of the 
name now has laboratories in which the actual operations are learned by the 
students in repeated efforts under the direction of expert teachers. Books are 
helps, and often give an expert all that he needs to enable him to perform 
successfully some difficult or unfamiliar operation. But the living teacher and 
the actual experiment serve the beginner most effectively. 

We strongly recommend the operator to possess the best works on Moving 
Pictures and projection in general, and to subscribe for one or more periodicals. 
By studying these he can keep himself informed of all the advances in his pro- 
fession. In the long run, the "man who knows" is appreciated. 

It was inevitable that with the exceedingly rapid development of the moving 
picture business many difficult operations, and the special form of acting 
requisite to the production and exhibition of a photo-play were undertaken by 
persons without adequate training and experience. It seems to the authors 
that it is highly creditable to human intelligence that the work has been so well 
done and that the improvement has been so constant and rapid. It seems to 
us, furthermore, that an important factor in the present creditable attainments 
which have already been reached, has been due to the high standards advocated 
by the Moving Picture World in all branches of the art. In particular the 
authors wish to commend the work of Mr. F. H. Richardson in his Motion 
Picture Handbook and in his weekly discussions and answers to questions in 
the projection department of the Moving Picture World. 

390 



CH. XI] MOVING PICTURES 391 

551. For the historical development of moving pictures see 
under History in the Appendix. 

For works on moving pictures see: Cyclopedia of Motion 
Picture Work, 2 vols.; Hepworth, C. M., Animated Photography; 
Hop wood, Living Pictures ; Jenkins, C. F., Handbook for Motion 
Pictures and Stereopticon Opera; Jenkins, C. F., Picture Ribbons; 
Richardson, F. H., Motion Picture Handbook, 2d ed.; Talbot, 
F. A., Moving Pictures; Hints to Operators by the Nicholas 
Power Company; Periodicals on Moving Pictures, e. g., the Mov- 
ing Picture World and catalogues of manufacturers and dealers 
in moving picture outfits. 



INTRODUCTION 

552. The steps that had to be taken in human experience and 
knowledge before it was possible to have moving pictures at all, 
were many ; and the time between some of the steps was very long. 

The first step was a knowledge of the physiology of vision, and 
especially a knowledge of the persistence of visual impressions. 
Primitive man knew that a glowing torch would make a circle of 
fire if it were whirled around rapidly enough. He knew also that he 
could see objects illuminated by an instantaneous flash of lightning. 

From this power of seeing by an instantaneous illumination, and 
the persistence of the impression for a limited time after the light 
has gone, arise the possibility of having moving pictures. In a 
word, moving pictures are possible because we can see instantly, 
but we cannot stop seeing instantly. 

To give views rapidly with proper illumination, involved the 
discovery of means for artificial light of great brilliancy, and of a 
machine by which the views could be lighted and moved along; 
and finally the long series of discoveries and inventions in optics 
and chemistry before photography was invented to make the pro- 
duction of the views cheap and accurate. It was another long step 
taken by Newton when he showed that white light in nature is 
composed of the rainbow colors. Furthermore, it was shown by 
him and contemporary and later physicists and physiologists that a 
mixture of less than the seven colors of the rainbow gave to the eye 
the appearance of white light. Even two complementary colors 



392 MOVING PICTURES [Cn. XI 

as red and greenish blue, yellow and indigo blue, etc., give the 
appearance of white. With this information it became possible 
to add to the photographic black and white moving pictures, the 
element of color. This was accomplished by using isochromatic or 
panchromatic film, and taking the pictures through colored screens, 
the first picture through a red, the second through a green, the 
third through a violet screen and this constantly repeated through- 
out the whole scene. In exhibiting the picture there is a three- 
color screen used so that the picture exposed through the red screen 
is projected through a red screen, giving a red image, and the other 
colors in like manner. If the film is run through the machine three 
times as fast as the black and white film, then the brain mixes the 
colors of the successive pictures giving fairly true color values and 
black and white. Where only two screens are used red and green 
the process is the same, but the film has to be run through the 
machine only twice as fast as the black and white film as there are 
but two colors for the brain to combine. Naturally the combina- 
tion of two colors gives a lower range of possibilities than the mix- 
ture of three colors, but even this is wonderful, as all will agree who 
have seen the colored moving pictures reproducing the gorgeous 
scenes of nature or the pageants of human splendor in all their 
form and movement and also with a fair approximation to the color 
effects. 

So perfect have become the materials and processes used in 
photography, and the accessory mechanical appliances, and the 
artificial lights available, that now the scientist can register 
accurately the almost instantaneous movements of an insect's wing, 
the flight of a cannon ball, and the numberless actions everywhere 
in nature which are so rapid that the unaided eye cannot analyze 
them. On the other hand, the movements in the processes of 
nature which are so slow that one can only see what has been 
accomplished in an hour, a day or a year, can be hastened by the 
moving picture machine so that the actual changes can be made to 
appear as if they occurred in a brief time, and the actual move- 
ments which were too slow for the eye to recognize, are made to 
appear rapidly enough for the eye to follow them. In this way the 



CH. XI] MOVING PICTURES '393 

actual movements in a growing plant or an opening flower are 
revealed to the eye ; and the great steps in the evolution of an egg 
to a complete animal, swimming, walking or flying, stand out with 
startling reality. 

The last triumph is the combination of the phonograph and the 
moving picture machine so that both the eye and the ear are 
appealed to as in real life or in the theater with living actors. 
This combination was suggested by Muybridge, the first to analyze 
and then combine the movements of animals by photography and 
projection. That suggestion was made in 1888, but it is only now 
after 25 years that a fair degree of success has been obtained. It 
requires two operators, one for the moving picture machine and 
one for the phonograph. The phonograph is just behind the 
screen, while the moving picture apparatus is in the usual place at 
the back of the theater. 

The screen is sufficiently transparent so that the phonograph 
operator can see the moving pictures, and the moving picture 
operator has telephonic connections with the phonograph so that 
he can hear accurately the sounds. He can, of course, see the 
moving pictures on the screen. The phonograph is made the 
master machine and the pictures must be made to follow the sounds. 
This is partly accomplished by a direct connection between the two 
machines, and partly by the intelligent cooperation of the two 
operators. 

The first successful efforts in moving pictures were made by 
physicists and physiologists who desired to analyze the complex 
and rapid movements of men, animals, and machines. The pur- 
pose was wholly scientific, but it was early seen that herein lay the 
possibility of entertainment and general instruction. 

The entertainment or amusement feature is, perhaps, now the 
predominant one; but the religious, educational, economic and 
scientific use of this powerful means for portraying action has never 
been lost sight of, and to-day is more prominent than ever. 

Much has been said and written on the moral or social effect of 
the moving picture. The writers and their friends have visited 
moving picture theaters in many cities and in many lands to see 



394 MOVING PICTURES [Cn. XI 

the kinds of scenes that were portrayed, and the kinds of people 
who crowded the theaters to see them. At the same time they 
have also visited the regular theaters to see actual human beings in 
the plays, and the kind of plays and the kind of people who were 
there to see them. 

To some of us, at least, the actual stage and the screen-stage 
seem equally real. The screen-stage has the advantage of a 
boundless, and untrammeled outlook of land and water, earth and 
sky in calm and sunshine and in the resistless action of storm or 
volcanic eruption. 

In human life it can show actual scenes, commonplace or heroic ; 
scenes like a royal coronation, or the barbarisms of war and riot, 
and on a scale impossible for a regular theater, and at an expense 
which makes them available for all mankind to see and enjoy, each 
according to his own knowledge, experience and capabilities. 

That some of the scenes in moving picture theaters are neither 
inspiring nor uplifting, and that the order in which the scenes 
appear is sometimes unfortunate, must be admitted. But these 
and all other defects which have been pointed out are not inherent 
in the moving picture. They simply indicate human failings. 
They can be corrected and are being corrected all the time. 

It is perfectly natural to think of the advantages to be gained by 
impressing moving pictures into the service of education. The 
striking scenes depicted by the moving picture are well adapted for 
arousing interest and giving the inspiration which lead to the care- 
ful and painstaking effort necessary for a true education. For 
example, in the development of a frog or a fish from the egg the 
moving picture shows the major changes but not the minor ones 
which are the really essential changes. No one would ever become 
an embryologist by looking at moving pictures of a developing 
animal or plant, and so with all the other subjects the study of 
which enters into an education. 

There are a good many helps in education, but there is no way 
to become really educated in any subject without the continuous 
and concentrated study of details as well as of the subject as a 
whole, any more than a man can become a skilled mechanic by 



CH. XII MOVING PICTURES 395 

simply visiting the best conducted machine shop in the world. 
Education is personal ; everything gained has to be paid for to the 
last farthing in mental effort. 

Moving pictures are the offspring of science through some of the 
finest minds that the world has known. It is simply for the finest 
art, the best science and the highest aspirations of mankind to take 
this powerful agent their offspring and put it to the real service 
of humanity. Let it do what it is so capable of doing in the church, 
in general and technical schools of all grades; in scientific, educa- 
tional and philanthropic societies; in the theater, in the club, and 
finally in the home. 

AUDITORIUM, SCREEN AND OPERATING ROOM 

First, it is necessary to consider the room for projection, its 
arrangement for seats, lighting during and between exhibitions, the 
screen and the position of the machine. 

553. Auditorium and screen. The auditorium should be 
arranged so that everyone in the room can get a good view of the 
screen, there should be a sufficient number of aisles and exits in 
order that the room can be filled or emptied quickly and without 
disturbance; and provision should be made for giving a sufficient 
illumination during the performance so that people can find seats 
or leave the room without difficulty. 

The screen should be dead white and free from wrinkles. If 
simultaneous sound effects are to be produced it is an advantage to 
have the screen slightly translucent so that the pictures can be seen 
from behind. In a long narrow room one of the metallic screens is 
an advantage. These screens are very poor for those on the side 
when used in a wide room, as the picture appears very dim when 
seen from the side. When the hall is provided with a stage it is well 
to hang the screen quite a distance from the front of the stage so 
that it will be easier to avoid stray light and in order that the people 
in the front seats will not be too close to the picture. A dark 
border or frame to the screen is also an advantage. (For the size 
of screen and of the screen images see Ch. XII, 633, 638-639). 



396 OPERATING ROOM [Cn. XI 

554. Position of the machine. The machine should be 
located so that its optic axis is perpendicular to the screen or the 
pictures will be distorted. If the machine cannot face the screen 
directly it is better to have it in the middle of the room and pointing 
upward or downward, or to have it at the same height as the screen 
and pointing slightly to one side. The worst possible distortion 
occurs when the machine is pointed obliquely downward as it must 
be when placed in one side of a gallery. 

555. Tent or booth for temporary operation. For a single 
performance the machine may be laid on a table in the middle of 
the auditorium just as with a magic lantern or it may be enclosed 
with a temporary booth or tent to enclose any stray light and to 
overcome the distracting effect of the machinery. 

556. Permanent operating room. Permanent installation 
should include an operating room large enough so that the machine 
or machines can be operated without hinderance or loss of time 
from lack of sufficient space. This is very essential in any place 
where even a short delay is so disagreeable to the audience. The 
operating room should be easy to get to and it should be well 
ventilated. It should have a large flue at least 50 cm. (20 in.) in 
diameter, connecting with the outside of the building. All open- 
ings in the operating room should be provided with shutters which 
will close automatically in case of fire. The room should be pro- 
vided with incandescent lamps and extension cords to use while 
working around the machine and finally there should be an electric 
fan and a chair for the operator. Every machine should be 
accessible from all sides. Film boxes should be placed where they 
can be easily reached. Sufficient tools for ordinary operation, a 
supply of carbons, pins, film cement, and extra condenser lenses 
should also be handy. A shop-room equipped for making repairs 
to the machines and for doing jobs of wiring should be near the 
operating room. It is not advisable to try to do such work in the 
operating room itself. 

The operating room is to be at all times kept like a battle-ship in 
time of war, with the decks cleared for action, nothing there which 
is not actually required. 



CH. XI] OPERATING ROOM 397 

557. Construction of a modern operating room. For the 

construction of the operating room itself a good description is given 
byF. H. Richardson in the Moving Picture World of August 12, 
1911, p. 372. See also Richardson's Handbook, pp. 65-93. 

(1) "No operating room may have less than 50 square feet of 
floor surface, or be less than seven feet, in the clear, from floor to 
ceiling at any point. 

(2) All operating rooms shall have a vent flue of not less than 
1 3/2 square inches area to each square foot of floor area, same to 
extend from the ceiling, or a point near the ceiling, to the open air, 
above the roof if possible ; provided, however, that no vent exceed 
360 square inches in area. 

(3) All operating rooms shall be of such fireproof construction 
as is approved by the National Board of Fire Underwriters or the 
City Fire Marshal. 

(4) Every operating room shall have a door, opening outward, 
not less than 2x6 feet in size, provided with an appropriate spring 
to hold same shut. 

(5) Every opening from operating room into auditorium, 
except door, shall be equipped with a metal shutter, sliding in 
grooves and semi-automatic in action. Same shall be so arranged 
that all shutters are held open by a single cotton master cord 
passing directly over front edge of upper magazine of each machine, 
just high enough to clear operator's head when standing. Shutters 
may close by their own weight or by force of a spring. If vent 
flue is provided with damper it shall be so weighted that it will 
normally stand open and shall only be held shut by cord attached 
by master shutter cord before mentioned. 

(6) Front, sides, and top of every lamp-house shall be tightly 
enclosed, except for vent-holes, protected by wire gauze screen, but 
back of lamp-house may be open. 

(7) All moving picture projection machines shall be equipped 
with approved upper and lower magazines, doors of which shall be 
closed when machine is running. 

(8) All rheostats shall be located outside the operating room, 
but low voltage transformers (inductors, economizers, etc.), used 
to control the current may be located inside the room. 



3Q8 MOVING PICTURE APPARATUS [Cn. XI 

(9) No wire of less size than No. 6 B & S gauge shall be used in 
any projection arc circuit. 

(10) Only link fuses, enclosed in suitable metal cabinet with 
spring door, shall be allowed in any operating room. 

(n) All wires, except asbestos covered from outlet to lamps, 
shall be in conduits. 

(12) All switches shall be enclosed (fig. 278). 

(13) All carbon butts shall be deposited, immediately on 
removal from lamp, in metal can containing water. 

(14) All films shall be kept in solderless metal case with ap- 
proved spring-closing cover, or door. 

(15) Smoking shall be absolutely prohibited inside the operat- 
ing room. 

(16) There shall be no reading matter inside any operating 
room. Reading matter to be construed to mean newspapers, 
novels, etc., but not including catalogues, or books of instruction, or 
magazines helpful to the operator in his work. 

(17) Not to exceed one ounce of alcohol or one pint of lubricat- 
ing oil shall be allowed in the operating room. Benzine, kerosene 
and like substances shall not be kept in any quantity in any theater. 

(18) Machines may be motor driven. 

(19) All machines shall be firmly and effectively anchored to 
the floor." 

558. Source of electric current. Next to be considered is the 
source of current supply. If one is in a place where there is a good 
electric system in operation it is usually much better to buy the 
power than to try to run a special power plant. This is because of 
the greater certainty of the city power and the absence of responsi- 
bility. It is, however, perfectly feasible to generate power with a 
gasoline, oil, alcohol or steam engine. When this is done the power 
is somewhat cheaper but rather more trouble and without careful 
attention it is less certain than a regular supply. Independent 
generation of power in small units makes possible the direct con- 
nection of the arc to the generator without the use of a rheostat as 
will be explained later. (Ch. XIII, 680, see also 562). 



CH. XI] MOVING PICTURE APPARATUS 399 

559. Wiring. When the supply is decided upon, the wiring 
is next installed. This must be heavy enough to carry the greatest 
current which is to be used continuously in the lamp. It does not 
need to be designed for the rather high current which flows when 
the carbons are brought into contact, as any wiring can withstand 
a heavy overload for a few seconds without injury. 

The wire which enters the lamp-house should be flexible cable, 
asbestos covered, and of a carrying capacity at least double the 
amount required for use at the arc ( 694-695) . This is on account 
of the high temperature within the lamp-house and consequent 
rapid deterioration of a small wire. 

560. Fuses. Fuses should be used in every case and not 
circuit breakers. This is because a fuse will not "blow" instantly 
when current is drawn greater than its normal capacity (as when 
the arc is started) but if this overload is continued, it will melt and 
open the circuit. The circuit breaker, on the other hand, will open 
the circuit instantly at the same amperage whether the current is 
momentary or long continued. 

561. Fire underwriters and special regulations. The wiring 
and installation must conform to the fire underwriters regulations 
and any special requirements of the city in which the theater is 
located. The wiring for moving picture machines is neither 
heavier nor more difficult to install than that required for other 
forms of projection, notably opaque projection, provision for 25 
amperes direct current or 50 amperes alternating current usually 
being sufficient for small theaters. 

For currents required in different cases; for the size of wire 
required for these currents and for fire underwriters regulations see 
Chapter XIII, 691. 

562. Rheostat or other ballast. As with all forms of arc 
lamp, the moving picture lamp requires some form of ballast or 
regulating device to control the current. 

The simplest and cheapest device is of course the resistor or 
rheostat. When the electric supply is no volt direct current, a 
rheostat is generally used. A rotary motor-generator set or 



400 



MOVING PICTURE APPARATUS 



[On. XI 



"current saver" is sometimes used as a ballast with direct current 
and effects a considerable saving of power especially when the 
supply is 220 or 500 volts. (See Ch. XIII, 744). 

When the supply is alternating current the ballast may be in the 
form of a rheostat but reasons of economy exclude this form of 
ballast when the machine is used continuously. For continuous 
performance an inductor (choke-coil), a special transformer, a 
mercury arc rectifier or a motor-generator is used. (See Ch. XIII, 
682-683, 72 




FIG. 221. EDISON KINETOSCOPE, PROVIDED 

WITH TWO-WING OUTSIDE SHUTTER. 
(Cut loaned by the Edison Manufacturing Company). 

When power is independently generated, a special dynamo can 
be connected directly to the arc lamp without ballast, the dynamo 
will be its own regulating device. (See Ch. XIII, 680). 

Whatever form of ballast is used, the quality and workmanship 
should be of the best or an endless amount of trouble may be 
expected. The rheostat or other ballast must conform to the 
underwriters regulations and must be satisfactory to the company 



CH. XI] MOVING PICTURE APPARATUS 401 

supplying the power. Some power companies object to the use of 
an inductor (choke-coil). In such cases a transformer can be used 
instead. 

563. Stand or table. A stand or table is provided by the 
makers of the machine. The method used to set up the stand will 
be fairly obvious from the illustrations furnished by the makers of 
the particular machine used. Generally this stand is made of 
brass tubes. One maker provides a heavy iron pillar. With this 
make provision must be made to anchor this pillar firmly to the 
floor. 

If the machine is to be installed permanently, it is often better to 
use a stand constructed of concrete or a very heavy wooden table 
instead of the light stand regularly supplied. A very slight motion 
of a rickety stand will cause an enormous movement of the picture 
on a screen 15 to 30 meters (50 to 100 feet) away. 

564. Unpacking. The moving picture machines coming from 
the factory are very carefully packed. When removed from the 
box, it is advisable to take careful notes of just how the different 
parts are packed and to number the wooden cleats used to hold 
things in place, especially if the machine will need to be shipped 
away again. 

Be careful in unpacking all parts, especially the lenses. Do not 
throw away any wrapping material until sure that no parts are 
missing. 

565. The moving picture machine. When unpacked the 
moving picture machine will be found to consist of a stand and base- 
board, arc lamp, lamp-house, condenser, aperture plate, objective, 
shutter, film magazines, and mechanism for moving the film. 
There will also be an extra film reel and a rewinder (fig. 221-224). 

566. The arc lamp. The arc lamp usually supplied with 
moving picture outfits is of the hand-feed type with inclined car- 
bons. The handles for feeding the carbons and for slight up and 
down adjustments project backwards so they may be manipulated 
without opening the lamp-house. The good makes of arc lamp are 
adjustable so that the carbons can be held in the vertical or the 



402 MOVING PICTURE APPARATUS [CH. XI 

inclined position as desired and each carbon holder can be turned so 
that the upper carbon is inclined and the lower one is vertical 
The right-angle arc can be used with the moving picture outfit if 
desired, but it should not be used with currents much above 25 
amperes Twenty-five amperes direct current will be found 
sufficient for all but the largest rooms. 




Fic^222.. POWER'S CAMERAGRAPH No. 6, SHOWING THE LAMP-HOUSE 

IN POSITION 
(Cut loaned by the Nicholas Power Co.). 

567. Lamp-house. The arc lamp is enclosed by a metal 
house to protect the operator from being blinded by stray light and 
to protect the arc from air currents which might blow it out or 
otherwise interfere with its performance. The adjusting handles 
of the lamp project so that the lamp can be adjusted from time .to 
time without opening the doors of the lamp-house. 

The house should be provided with doors to enable the operator 
to change the carbons and should have a window of dark glass so 



CH. XI] MOVING PICTURE APPARATUS 403 

that the arc can be watched. This window should be of fairly 
large size and directly opposite the crater of the arc. The glass 
should be dark enough so that the eyes will not be tired by the too 
great brightness and yet light enough so that the whole of the hot 
carbon ends can be seen. 

Another convenient way to observe the arc is to bore a fine hole 
in the side of the lamp-house away from the operator. This acts 
like apinhole camera and an image of the arc is seen on the opposite 
wall. A sharper image of the arc can be formed by using a long 
focus lens over an opening in the wall of the lamp-house to focus an 
image of the arc upon the wall. A spectacle lens of about 25 cm. 
(10 in.) focus (4 diopters) will answer. The lens may be held by 
any convenient clamp but must be adjusted for distance to get the 
sharpest image, otherwise it is no improvement over the simple 
pinhole. 

The lamp-house should be well ventilated as from ^ to 2 kilo- 
watts of power, .7 to 3 horsepower, is converted into heat. While 
the arc is going there must be some way for this heat to escape, 
otherwise everything inside would melt. One of the principal 
causes of condenser breakage is poor ventilation of the lamp-house. 
The best ventilation is secured by having holes permitting air 
circulation but no escape of light, at the top and near the bottom 
of the lamp-house. The back of the lamp-house is sometimes 
removed.' 

In many places the fire underwriters or the city, require that 
these ventilating holes be covered with fine wire gauze, to prevent 
sparks flying out. This requirement was invented by someone 
who had the mistaken idea that an arc lamp was a fiery volcano, 
vomiting out sparks and lava in all directions instead of a quiet, 
well behaved sort of thing. It is true that a minute spark some- 
times does fly up, but it is so light that it cannot do any damage. 
Any small piece of the carbon tip which breaks off will fall to the 
bottom of the lamp-house where a suitable tray should be pro- 
vided to catch it. This tray is also useful to hold the short pieces of 
hot carbon just taken out of the lamp when new carbons are put in. 

568. Condenser. The condenser is usually in a box which is 
fastened to the lamp-house and moves with it. In front of the con- 



404 MOVING PICTURE APPARATUS [Cn. XI 

denser is the lantern-slide carrier for use with the magic lantern, 
which is usually found in connection with moving pictures. 




FIG. 223. NEW STYLE CONVERTIBLE BALOPTICON WITH POWER'S 

MOVING PICTURE ATTACHMENT. 
(Cut loaned by the Bausch & Lomb Optical Co.}. 

The condenser is usually provided with two plano-convexlenses, 
each of 18 or 19 cm. focus (7 to ;> in. focus). JPP^ 

The slide-carrier for the magic lantern usually found connected 
to the moving picture outfit is generally fastened to the lamp-house 



CH. XI] 



MOVING PICTURE APPARATUS 



405 



directly in front of the condenser. This is not a good plan as it 
cuts down the light from the condenser, and as the opening is not 
round but a quadrangle it often leads to queer shadows on the 
screen. Some makers provide a stationary slide-carrier opposite 
the magic lantern objective so that the whole face of the condenser 
is free when it is opposite the moving picture objective ; this is a 
better method than the above. 




FIG. 224. DOUBLE DISSOLVING MODEL C BALOPTICON WITH EDISON 
MOVING PICTURE ATTACHMENT. 

(Cut loaned by the Bausch & Lomb Optical Co.). 

569. The moving picture head. This contains all of the ele- 
ments of the moving picture machine except the arrangement for 
lighting. The moving picture head holds the objective and con- 
tains the film-moving mechanism and the aperture plate. 

570. Aperture plate. Considered optically the aperture plate 
which serves as a frame for the picture on the film is the most 
important part of the moving picture head. 



406 MOVING PICTURE APPARATUS [Cn. XI 

The standard aperture plate has an opening 23.08 mm. wide x 
17.31 mm. high (29/32 in. x 87/128 in.) with rounded corners. 
When the picture is in focus on the screen the edges of the aperture 
plate are also in focus at the same time ( S7oa). 

571. The objective. The objective forms the image of the 
film picture upon the screen. It is in design exactly like an objec- 
tive for the magic lantern but is of shorter focus. 

It is better to have the lenses of large diameter (see 830). 

Moving picture objectives with lenses 45 mm. (iK in.) and 
65 mm. (2^2 in.) in diameter are on the market. The objectives 
45 mm. (1^4 in.) in diameter will answer but those of 65 mm. (2^2 
in.) are to be preferred. The larger objectives will give with less 
trouble a screen image without shadows. (See 829, 830). 

One must select an objective of suitable focal length to give a 
proper sized screen image for the auditorium to be used. This is 
dealt with more fully in 635. In most rooms a screen image of 
suitable size will be obtained with an objective of between 12.5 to 
13.5 cm. focus (5 to 5 y in.) when the moving picture machine is at 
the back of the room. 

572. The film mechanism. This consists in the proper gears 
and sprocket wheels for moving the film, and for turning the shut- 
ter. The mechanism is complex; differs in different makes of 
machines, and no attempt will be made here to describe it in detail. 

573. The shutter which cuts off the light during the time 
when the film is in motion is located either just beyond the aperture 
plate and hence before the objective (fig. 225), or just beyond the 
objective (fig. 226, 227). When located between the aperture 
plate and the objective, it is called an inside shutter and when 
located beyond the objective it is called an outside shutter. 



570a. Standard aperture. As there was some lack of uniformity in the 
size of the opening of the aperture plate, the Gundlach-Manhattan Optical 
Co. has selected a size for a standard as follows : The aperture has an opening 
of 23.08 mm. long and 17.31 mm. high (29/32 x 87/128 inch). This standard 
has been adopted by the Nicholas Power Co., the Edison Co., and the Precision 
Machine Co. No doubt the other makers of machines will adopt the standard 
in due time. Moving Picture World, Vol. 20, April 1 1, p. 210, April 25, p. 512. 



CH. XI] 



MOVING PICTURE APPARATUS 



407 



574. The film magazines are large sheet iron.: boxes which 
hold the film reels. They are big enough to hold the standard 
25 cm. (10 in.) reel and it is a convenience if they are large enough 
to hold the larger reels of 30 cm. (12 in.) diameter. The film 
magazines are fitted with fire traps to prevent any fire getting into 
the magazine if the film should start to burn. 




FIG. 225. MOVING PICTURE MECHANISM WITH INSIDE SHUTTER, I S. 
For full explanation see Fig. 231. 

INSTALLATION OF A MOVING PICTURE OUTFIT 

575. After the wiring to the operating room has been installed 
in accordance with the fire underwriters regulations and any special 
regulations of the city in which the work is done, all is ready to 
connect in the rheostat, transformer, or other regulating device 
( 7 2 8, 736) and to attach the wires to the arc lamp. 

These connections are exactly like those for the magic lantern 
(fig. 3) when a rheostat or inductor (choke-coil) is used. When a 
transformer or mercury arc rectifier is used, the primary side is 



408 



MOVING PICTURE APPARATUS 



[CH. XI 



connected to the line, and the secondary side is connected to the 
arc lamp. (See Ch. XIII, 683, 739). 

The switches should be in a convenient location, so that the 
current can be turned on or off without moving from the operating 
position. 

As soon as the connections are made it is well to use an ammeter 
and to find what current the arc will draw with the different set- 
tings of the controlling lever of the rheostat or transformer. It is 




FIG. 226. MOVING PICTURE MECHANISM WITH OUTSIDE SHUTTER, O S. 
For full explanation see Fig. 23 1 . 

a good thing also to use a voltmeter to determine the line voltage 
on open circuit, also the voltage across the line, between the arc 
terminals, across the rheostat or choke-coil, or if a transformer is 
used, the voltage given by the secondary both on open circuit and 
when the arc is running. The voltmeter or ammeter must be 
designed for the kind and amount of current for which it is to be 
used, that is, alternating or direct current. When a rectifier or a 
motor-generator is used it will be necessary to have both direct 



CH. XI] OPTICS OF MOVING PICTURES 409 

current and alternating current instruments. (See Chap. XIII 
for using these instruments 662-674). 



OPTICS OF MOVING PICTURE PROJECTION 

576. For purposes of description the projection of the 
individual pictures of a film can be considered apart from the 
mechanism which moves the film. 

The projection of the film picture has much in common with that 
of the ordinary lantern slide but it is somewhat more difficult. 

A theoretical treatment of the proper method of lighting the film 
is found in 825. Briefly stated it is this: Light from the arc is 
collected by the condenser so as to illuminate the film. This 
illumination must be very intense and at the same time must be 
evenly distributed over the entire area of the film. To secure this 
result with the ordinary large condensers (4^2 in. in diameter) 
requires the condenser to be quite a distance away from the film, 
the crater of the arc to be of considerable size, and the projection 
objective to be of fairly large diameter. 

Fig. 228 shows the optical arrangement most commonly used. 
Light from the arc is collected by the condenser upon the 
film at s, passing through the transparent parts of the film, it is 
bent by the objective in such a way as to form a sharp image of 
the film s, upon the screen. 

Only one picture of the film is seen at a time, the rest being 
carried in the magazines or covered with shields. The picture 
to be shown is just in front of the opening of the aperture plate. 
Optically we are concerned only with the aperture plate and the 
short section of film behind it. It is this short section of film which 
must be evenly illuminated and projected upon the screen. 

Beyond the film is the objective (fig. 229). The objective should 
be of good quality as it is the objective which determines the 
sharpness of the screen picture. Moreover, the objective must 
not be of too small diameter, for if it is too small there is danger 
that the screen image will not be evenly lighted although the 
illumination of the film may be perfectly even. The focal length 



4io 



OPTICS OF MOVING PICTURES 



[Cn. XI 




FIG. 227. MECHANISM OF POWER'S No. 6 CAMERAGRAPH, SHOWING THE 

THREE-WING, OUTSIDE SHUTTER. 
(Cut loaned by the Nicholas Power Company). 

of the objective determines the size of the screen picture for a given 
screen distance. 

577. Lining up the moving picture machine ; Adjustment of 
the light. The machine being assembled on the board, the parts 
lined up mechanically as well as possible ( 51 +), the final steps to 






CH. XI] 



OPTICS OF MOVING PICTURES 



411 




get a good light on the 
screen must now be taken. 
The moving picture head 
as it comes from the fac- 
tory should have the 
aperture plate and the 
center of the objective 
mount at the same 
height. If they are not, 
the aperture plate must 
be moved up or down 
until its center is on the 
same axial line as the ob- 
jective. The adjustment can prob- 
ably be done with sufficient accuracy 
with the eye, when looking through 
the lens opening, the lens being in 
place. This is a matter which al- 
ways should be looked after by the 
manufacturer. 

Another method would be to re- 
move the condensers and adjust the 
arc lamp to exactly the same height 
as the aperture plate. A piece of 
paper put in place of the lens 
should, when the arc is lighted, 
show the shadow of the center of 
the aperture plate in the exact cen- 
ter of the circular piece of paper. 

FIG. 228. OPTICAL SYSTEM AND ILLUMIN- 
ATION OF MOVING PICTURES. 

Lamp. 

Condenser. 

S Lantern-slide holder. 

Fire Shutter This is open only when 
the machine is running. 

s Aperture plate. 

Objective. 

See fig. 231 for full explanation of the 
mechanism. 




412 OPTICS OF MOVING PICTURES [Cn. XI 

The center of the condenser and the center of the aperture plate 
are adjusted to the same height above the baseboard. This is 
attended to by the manufacturer, but if a head of one machine is 
used with the arc and condenser of another make, adjustment 
might have to be made. Make a spot with a pen or a wax pencil 
exactly in the center of the front lens of the condenser, measure the 

height of this above the baseboard. 
Make a similar mark on the aperture 
plate at the height of the middle of 
the opening and measure its distance 
above the baseboard. If the aperture 
plate is too low the head should not 
be screwed directly to the baseboard 
but should be lifted U P sufficiently with 
(Cut loaned by the Gundlach- a thin piece of board. If the aperture 
Manhattan Optical Co.). plate is too high( the front of the base . 

board can be cut down or the lamp-house and condenser can be 
raised by using a piece of wood or asbestos board between the base- 
board and the lamp-house fastenings. 

After getting the objective, aperture plate, and condenser at the 
correct height, it only remains to get the arc at the right height. 
This is done from time to time by raising or lowering the arc lamp 
until the light spot falls exactly over the aperture plate. 

The sidewise adjustment of the lamp-house is now made in the 
same way by measuring the distance from the edge of the base- 
board to the center of the condenser and then to the center of the 
aperture plate. This measurement can be made by using a vertical 
board (52). 

When the same arc and condenser are used for both moving pic- 
tures and lantern slides, the lamp-house should be in the correct 
position when pulled on its lateral rods as near as possible to the 
operator. If it is not, stops can be fastened on the side rods to hold 
the lamp-house in the correct position. 

578. Back and forth adjustment of the arc lamp and con- 
denser. This is one of the most important and troublesome adjust- 
ments to make. There would be but little difficulty in getting an 



CH. XI] OPTICS OF MOVING PICTURES 413 

even illumination of the film picture and the screen image if con- 
densers were obtainable entirely free from spherical aberration, 
but this is not practical. No rule can be given as to the best 
position of the arc and condenser but the best position must be 
determined for each particular case. Some general hints can, 
however, be given. 

First The objective should be of large diameter. This will 
allow of a greater range of adjustment through which good illumin- 
ation can be obtained ( 829-830). 

The lenses of the double-lens condenser (fig. i) should be as near 
together as possible without actually touching. The convex sides 
of the condenser lenses should face each other, the plane sides 
should face the lamp and the objective. 

The condenser lenses should be of fairly long focus, 18 to 19 
cm. (7 to y>2 in.). 

If the condenser is as far away from the aperture plate as 
possible the illumination is usually more even, though less intense 
than when the condenser is close to the aperture plate. 

When first setting up the machine, it is a great help to have a 
series of condenser lenses to try, say such a series as two lenses of 
14 cm. focus, one each of 15, 16, 17 cm., two of 19 cm. focus, (two 
of 5^" in., one each of 6, 6>^, 7 in., two of 7^ in.). The two con- 
denser lenses should be of the same focus, then only one kind of 
condenser lens will need to be kept in stock to supply breakage. 

When the adjustment for distance is to be made, move the lamp- 
house with its condenser close to the aperture plate, fasten it 
in position, move the arc in the lamp-house nearer to and farther 
from the condenser until the best light is obtained on the screen. 
Note how this light appears and whether there are any ghosts or 
shadows. Then fasten the lamp-house and condenser slightly 
farther from the aperture plate and move the arc until the best 
light is again obtained. After repeating this, for every position 
of the condenser, the condenser is set at the distance which was 
found to be best. It may be necessary to try a different set 
of condenser lenses before the best possible result is obtained. 
This is a rather tedious process but is well worth while doing. 



414 MAGIC LANTERN AND MOVING PICTURES [Cn. XI 

ADJUSTMENT OF THE MAGIC LANTERN ATTACHMENT FOR USE IN 
CONNECTION WITH THE MOVING PICTURE MACHINE 

579. The adjustment of the arc lamp and condenser for the 
moving picture part is of much greater importance and is more 
difficult than that for the magic lantern attachment, hence, no 
attention should be paid to the projection of lantern slides until the 
projection of moving pictures is perfect. 

In most outfits the lamp-house moves sidewise on some lateral 
rods. When pulled towards the operator the lamp is in line with 
the moving picture objective, and when pushed away from the 
operator until it hits a stop, it is in line with the lantern objective. 

Push the lamp-house on these lateral rods until it is held by the 
stops. A lantern slide is put in the holder and the lantern objec- 
tive support is loosened and the lantern objective moved sidewise 
until it is over the spot of light from the arc and moved back and 
forth until the image of the slide is in focus on the screen. If there 
are shadows on the screen not due to malposition of the carbons, 
use an objective with larger lenses. 

If the lantern picture does not occupy the same place on the 
screen as the moving picture it may be the fault of the side adjust- 
ment of the slide-holder, or it may be due to faulty alignment of the 
arc lamp and moving picture head. If this should be the case 
move the lamp-house sidewise until the lantern-slide picture 
occupies the proper position on the screen. Then move the arc 
sidewise until the screen is well lighted and clamp it in position. 
When, now, the lamp-house is pulled into position in front of the 
moving picture objective the spot of light may not fall upon the 
aperture plate but to one side. If it is not in the right position do 
not alter the adjustment of the arc lamp but move the lamp-house 
as a whole to one side until the spot exactly covers the aperture 
plate. Then fasten the stop, so that the lamp-house will always 
occupy the same position when pulled toward the operator. 

580. Management of the arc lamp. During an exhibition it 
is necessary to watch the arc lamp to see that it is burning prop- 
erly. There are several ways of burning the arc which will give a 
good light : 



CH. XI] MAGIC LANTERN AND MOVING PICTURES 



415 



The carbons may be at right angles (fig. 23 C). 

The carbons may be inclined backwards about 30 (fig. 230 a). 

The upper carbon may be inclined backward 45, the lower 
carbon being vertical (fig. 230 c). 

The carbons may come together in the form of a horizontal V 
with the point towards the condenser (fig. 23 D). 

Both carbons may be vertical (fig. 230 b). 

Whatever carbon setting is used, the arc must be held, so that the 
crater or craters face the condenser. 

The form of the arc can be watched by observing it through the 
smoky glass window or by the pinhole or lens image on the wall 
( 567). When using alternating current the sound will give an 
indication as to whether the arc is of the right length. 

Constant vigilance in watching the arc is one of the requirements 
for success is showing moving pictures. During an exhibition, 
never let the arc go out. 

581. Supply of carbons for the arc lamp. A good supply of 
carbons should be provided and placed where they may easily be 
reached. The carbons are soft-cored and their size should be 
suited to the current used (see 753a). Generally 16 mm. carbons 
(^ in.) are used, both being of the same size. 

582. Position of the film in the machine. When a film is 
passing through the machine the rule for its position is the same 
as with the lantern slides, that is, the picture should appear correct 
when one looks through it toward the 
screen but it must be upside down. 
To accomplish this one should bear in 
mind that as the films are printed they 
will appear correct when one looks at 
the emulsion side just as with a lan- 
tern slide or an ordinary paper print. 
Therefore, the light is made to strike 
the emulsion side of the film. 



\ 



\ 



\ 



583. Mechanism. Without go- 
ing into the details of the special 



FIG. 230. POSITION OF CAR- 
BONS FOR MOVING PIC- 
TURE PROJECTION. 

a Inclined. 
b Vertical. 

c Upper carbon inclined, 
lower carbon vertical. 



MOVING PICTURE FILM AND MECHANISM [Cn. XI 



arrangements employed in the different makes of machine, the 
principle is simple, although the mechanical problems in work- 
ing out these principles require much care. 




Objective 



D U 


y 


1 1 






iin 


111 1 


I 






FIG. 231. FIGURE TO REPRESENT THE PRiNciPLE7oF THE MOVING 

PICTURE MACHINE MECHANISM. " 
a b Sprocket wheels moving with uniform velocity. 

c Intermittent sprocket wheel which jerks down the^short section^of film 
between L and M. 

i Idlers to hold the film on the sprocket wheels. 

D Gate which holds the film in place in front of the aperture plate. 

F Upper film reel, unwinding. 

G Lower film reel, winding up. 

S Aperture plate. 

Objective. 



CH. XI] 



MOVING PICTURE FILM AND MECHANISM 



417 



The essential part of the mechanism consists in three sprocket 
wheels, a, b, and c, (fig. 231), the two wheels a and b move con- 
tinuously at the average rate at which the film is passing (30 cm., 
i foot, per second), and serve to unwind the film from the upper 
reel F and feed the film to the take-up reel G at a uniform rate. 
The sprocket wheel c, located between the other two, is inter- 
mittent in its movements, being stationary for about % of the time 
and being in rapid motion for about J"6 of the time. The effect 
is, that after the film has been in position for exposure on the screen 
this sprocket wheel jerks the small section of film between L and M 
forward to the next picture. In fig. 232 is shown one form of 
mechanism for causing the intermittent movement of the sprocket 
wheel. 

When the film is stationary it is projected on the screen by the 
objective, but during the short time when the film is in motion a 
shutter either before or behind the objective cuts off the light and 
prevents any blurring due to the movement of the picture. 

The films are made in such a way that if the pictures are right 
side up, the later picture will be below the earlier ones, but as in 
passing through the machine the pictures are upside down, the 
later pictures are above and it is necessary to move the film down- 
ward to bring the pictures on 
.fiMMtfAtefc*, the screen in due order. 

584. Threading the film in 
the machine. The film as 
wound on the reel usually is 
wound in the correct direction, 
so that the first pictures are on 
the outside. If this is not the 
case, the film must be rewound 
on another reel to reverse its 
direction. If the direction is 
correct the pictures will be up- 
side down when the film is in 

FIG. 232. INTERMITTENT MOVEMENT the machine, that is, when the 

OF POWER'S No. 6 CAMERAGRAPH. ,-, -, -, c 

,,, , , , , ., ... , , film is passing downward from 
(L ut loaned bv the Nicholas Power 

Company). F (fig. 231). 




4i8 



MOVING PICTURE FILM AND MECHANISM [Cn. XI 



Next, it is necessary to get the film right side out, otherwise, 
everything will be reversed and appear as if seen in a mirror, an 
especially troublesome state of affairs when titles or letters are 
shown. The side of the film which has the emulsion appears 
rough, the other side is smooth and shiny. The film often has a 
tendency to curl, the emulsion being on the concave side. The 
film is turned so that the rough, emulsion side bearing the picture 
is toward the light. When it is wound correctly on the reel, and 
the emulsion side is turned so it will face the light as the film 
unwinds, the reel of film is p'ut in the upper magazine. The end 
of the film is pushed through the opening in the magazine between 
the rollers of the fire-trap. This can best be done by using the 
index and middle fingers to hold the film. 




FIG. 233. EDISON KINETOSCOPE MECHANISM. 
(Cut loaned by the Edison Manufacturing Company) 

The magazine doors are open showing the film reels. 
The film is inplace ready to project. 



CH. XI] MOVING PICTURE FILM AND MECHANISM 419 

The gate D is then opened and the idlers, iii are pushed away 
from the sprocket wheels a, b and c. A sufficient length of film 
is unrolled from F to reach to the take-up reel G and the film 
is put under the sprocket wheel a, so that the teeth fit into the holes 
at the edges of the film. Care must be taken that the film goes 
over or under the sprocket wheels in such a way that as the crank 
is turned forward all of the sprocket wheels tend to move the film 
in the same direction, otherwise they will tear it apart. The 
arrangement may differ in different machines. 

After putting the film on the sprocket wheel a, so that the teeth 
pass through the holes of the film, the idler i, is pushed over to hold 
the film in place. This can be done with one of the fingers while 
holding the film in place with the thumb and forefinger. The film 
is then engaged with the lower sprocket wheel b, leaving an extra 
length of film to form the two loops L and M. This can best be 
determined by experience, it must be enough so that the inter- 
mittent sprocket will not jerk the film in two and not long enough 
so that the loops will strike any shields there may be to cover them. 

The film is held against the intermittent sprocket c, so the loops 
L and M, are about equal in size and held straight on the tracks of 
the aperture plate when the gate D, is closed. 

The end of the film is now pushed through the fire-trap opening 
in the lower magazine and fastened to the take-up reel G. This is 
accomplished by slipping the end under the spring on the spindle 
of the reel, in such a direction that the film will not be folded as the 
reel is turned. The reel is turned to insure the end of the film 
being well fastened. Fig. 233 shows a mechanism with the film in 
position and ready to operate as soon as the magazine doors are 
closed. 

If the picture is not directly in front of the aperture plate but is 
above or below (misframed) , it can be put in its proper position by 
a lever which lowers the mechanism and film without disturbing 
the position of the aperture plate and objective. 

585. Direction of motion. The normal direction of motion 
to secure the proper sequence of events in the order in which they 
occurred is secured by moving the film downward, and results 



420 MOVING PICTURE FILM AND MECHANISM [Cn. XI 

from a right-hand rotation of the crank. If the crank is turned to 
the left the film will be pushed upward by the intermittent sprocket 
instead of being pulled downwards as it should be. This would 
most likely result in crumpling and breaking the film. 

586. Operation and speed. After the machine is threaded 
the lamp is pulled toward the operator so that the light shines upon 
the aperture plate. 

In starting the machine do not start with a jerk but start grad- 
ually (1-2 seconds), otherwise an unnecessary strain is put upon the 
gears. The crank is turned in a right-hand (clockwise) direction 
at such a speed that the film passes at the rate of 16 pictures per 
second. If the gearing is arranged so that the intermittent 
sprocket would move 16 times for each revolution of the crank, this 
would require i revolution per second or 10 revolutions of the 
crank every ten seconds. One should practice the speed for a 
while with no film in the machine, looking at the second hand of a 
watch and turning with a uniform speed until there are just 10 
revolutions every time the second hand passes a ten second division. 
This should be practiced for some time until the proper speed can 
be maintained with certainty. After the film is in, the action in 
the scene will serve as a guide for the proper speed, as some films 
are improved by being shown at a slower or faster rate than they 
were taken, i. e., the standard given above. 

See Richardson's Handbook, p. 219. 

587. Automatic fire shutter. As the machine starts, the 
automatic fire shutter (fig. 228) opens and allows the light to fall 
upon the film. If the picture is not at the right height on the 
screen it can be "framed up" by moving a lever which raises or 
lowers the mechanism and film. 

If an old machine is used that does not have an automatic fire 
shutter, one must be extremely careful never to allow the light to 
fall upon the film except when it is in motion, otherwise one or two 
seconds will suffice either to ruin the film if non-inflammable film 
is used or to start a conflagration if celluloid film is used. The 
danger from this source is so great that we strongly recommend 



CH. XI] MOVING PICTURE SHUTTER 421 

that a water-cell be used ( 848) in cases where an automatic fire 
shutter is not provided; where a motor is used to drive the 
machine; for all experimental work and for every person running 
a moving picture machine who has not had abundant experience in 
operating. It is so easy to let the film stop for a second, or to have 
the film break leaving a tag end of film in the aperture plate, and 
wonder afterward what started the fire. 

588. Setting or "timing" the shutter. The shutter should 
be mounted on the spindle used to turn it in such a way that it will 
cut off the light from the screen during the time when the film is in 
motion. If the shutter is not set exactly right in the beginning it 
is often a rather tedious job to correct its position, but by going at 
the matter systematically the difficulty is greatly lessened. 

Shutters of the one-wing type can, of course, be set in only one 
way but shutters of the two- or three-wing types may have wings of 
different widths. In this case the widest wing is the one which 
should intercept the light while the film moves. 

The easiest way to set the shutter would, of course, be to run 
the machine very slowly and watch the picture on the screen. If 
no shutter were used the picture would seem to jump up, and be 
replaced by a picture which comes up from below. When the 
shutter is in place, if the picture seems to jump up just before the 
light is out, the shutter is said to be too "late" and it must be 
loosened on its shaft and turned slightly forwards, that is, in the 
direction in which it is turning. The shutter is then fastened 
securely in position. If, the picture jumps into place from below 
just after the light comes on, the shutter is said to be too "early" 
and it must be turned slightly backwards. That the shutter may 
be correctly set when it is turning rapidly as well as when it is 
moving slowly, it is well to hold the outside of the shutter or the 
shaft on which it turns with the finger so as to take up lost motion. 
When in rapid rotation all the lost motion is taken up on account 
of air friction. 



587a. With a two-lens condenser the water-cell can be put between the 
condenser and the aperture plate (fig. 206). 



422 MOVING PICTURE SHUTTER [Cn. XI 

Running the machine slowly with a film in the machine is entirely 
practical provided the arc current is not extremely heavy, and 
provided a water-cell is used (See 596, 779-782). 

When no water-cell is at hand the machine must be run rapidly. 
In this case the rule for changing the position of the shutter is 
exactly the same but the motion of each individual picture cannot 
be seen. If one has a film which is nearly opaque, but has a few 
spots in it, as a period on a title for example, there is an effect 
known as "travel ghost" which is seen if no shutter is used or if 
the shutter is incorrectly timed. This is caused by the persistence 
of vision. As the white spot moves upward, it appears to be a 
streak instead of a spot. If, now, the shutter is too late, the light 
is not cut off until the spot starts to move upwards and a streak is 
seen above the spot. If the shutter is too early, the light is turned 
on while the spot is still moving upward and before it comes to 
rest; the streak is then seen below the spot. 

If the shutter is too narrow the motion of the spot, both before 
and after the light is cut off and the streak will be seen both above 
and below the spot of light. 

589. Rule for setting or timing the shutter. If the streak or 
travel ghost appears above the letters of the title, the shutter is too 
late, turn it slightly forward on the shaft. 

If the streak or travel ghost appears below the letters of the title 
the shutter is too early, turn it slightly backwards on the shaft. 

If the streak or travel ghost appears both- above and below the 
letters of the title, the shutter blade is too narrow.. Use a shutter 
with a wider blade. 

590. The best position of the shutter and the speed to prevent 
flicker. The shutter may be placed in either of two positions ; it 
may be just beyond the film and between it and the objective 
(inside shutter) or it may be placed beyond the objective (outside 
shutter). There is a difference in the effect produced depending 
on which of these positions is chosen (fig. 225-226). 

With the inside shutter, when the machine is turned slowly the 
image of the shutter can be seen somewhat out of focus traveling 
from one side of the picture to the other. 



CH. XI] FLICKER WITH MOVING PICTURES 423 

With the outside shutter, beyond the objective, the wing of the 
shutter as it advances removes light from the whole of the picture, a 
phenomenon which tends to reduce flicker. 

The diameter of the inside shutter is limited by the size of the 
mechanism, while the outside may be made as large as is desired. 

As will be seen below, the diameter of the shutter has an effect 
on the light. 

The picture should be entirely covered by the shutter before 
it commences to move, and it should not be uncovered until it has 
ceased to move. This requires that the wings of the shutter need 
to be about 3 cm. (i^ in.) wider than the theoretical J^6th of the 
circumference of the circle. 

The larger the circle the nearer to Yd of the circle is the width 
of the shutter wing. 

With a shutter of large diameter, the actual velocity is greater 
and the interruption of the light is more sudden, therefore a 
shutter of large diameter is to be preferred. 

591. Flicker. The standard speed of the film is given as 
18 meters (60 ft.) per minute, 30 cm. (i ft.) per second. There 
being 16 films per 30 cm. (foot), this gives 16 pictures per second. 

It is the general intention to run films at this speed though they 
are often run either faster or slower to get the best effects. The 
time during which one picture is shown (Vie second) can be divided 
into 6 equal periods, during five of these periods the picture is 
stationary and during the 6th the film is moved and the next 
picture substituted. 

One complete change will be called a cycle. 

The films could be run through the machine with no shutter at 
all, the film being in place an instant and then moved and the next 
picture substituted by a quick movement. This will cause a 
spreading out of white patches into a vertical streak called travel 
ghost, and will also give a general gray appearance and lack of 
contrast to the screen image. 

To avoid this appearance some kind of a shutter is used to 
obliterate the pictures while the film is in motion. The shutter 
can be either translucent or opaque. 



424 



FLICKER WITH MOVING PICTURES 



[CH. XI 




00 I 234 

Logarithm of ILLumination 

FIG. 234. THE RELATION BETWEEN THE ILLUMINATION OF THE SCREEN 

AND THE NUMBER OF FLASHES PER SECOND AT WHICH FLICKER JUST 

DISAPPEARS. 

If the flashes are more frequent than indicated by the curve for a given 
illumination there will be no flicker, but if less frequent, flicker will be seen. 

The solid line represents the observation of T. C. Porter and the dotted line 
represents some rough observations made by the authors. 



CH. XI] FLICKER WITH MOVING PICTURES 425 

If the shutter is translucent the appearance during the change 
of pictures is that of a screen lighted to a uniform gray. This kind 
of shutter is not much used in practice as it has the disadvantage 
of slightly illuminating the parts of the screen which should be 
absolutely black. 

The opaque shutters were originally made to cover the picture 
during the time the picture was in motion, i. e., from % to Y> of the 
cycle, the rest of the cycle the screen was lighted, but this was 
found to give a very bad flicker. 

Recently to get rid of the flicker the shutters have been made 
with 2 or 3 opaque wings. 

With the one-wing shutter a cycle is made up with 

1 . Picture on the screen screen light Y* to % cycle. 

2. Picture changed screen dark Y* to Yt> cycle. 

There are 16 cycles per second. The average transmission is 
YI to % of the incident light. 

It has been found that with a one-wing shutter the flicker is 
nearly as troublesome when the opaque part is Yf> as when it is 
Yt of the shutter. To avoid this, extra dark wings are added to 
the shutter, the form with 3 wings being the best With a three- 
wing shutter a cycle is made up of : 

1. Picture on the screen screen light % cycle. 

2. Same picture on the screen but screen dark Yt> cycle. 

3. Same picture on the screen but screen light Yt> cycle. 

4. Same picture on the screen but screen dark Yb cycle. 

5. Same picture on the screen but screen light Yb cycle. 

6. Picture changed screen dark Y> cycle. 

The screen is dark Yz an d light Y* f the time : Transmission of 
incident light, 50%. 

Each picture is thrown on the screen three times before it is 
changed for the next. Thus, while there are 16 cycles per second; 
there will be 48 flashes per second. 

At this speed, 48 flashes, flicker will altogether disappear (See 
592). 



426 FLICKER WITH MOVING PICTURES [Cn. XI 

THEORY AND EXPERIMENTS ON FLICKER 

592. Experiments have been made to determine the speed at 
which flicker disappears, that is, the speed at which the eye is un- 
able to distinguish between a continuous and an intermittent 
light. 

These experiments show that at a certain speed the appearance 
of flicker disappears. This speed is practically the same for 
different people. 

As the speed is increased the flicker disappears for the center of 
the field of vision before it does for the edge. Thus, the light on a 
screen may not appear to flicker when looked at directly but it may 
appear to flicker when looked at out of the "tail of the eye." 

As the brightness of illumination is increased the appearance of 
flicker is increased and a higher speed is required for flicker to 
disappear. Thus, when showing a very dark film, the light may 
not appear to flicker at all, while with a very transparent film or 
no film at all the light may appear to flicker violently although the 
speed is the same. 

If, instead of having the dark period and the light period equal, 
either the dark period or the light period is made less in proportion 
the flicker appears less violent, and it disappears entirely at a lower 
speed. This effect is, however, not very great. 

Thus, the flicker with a shutter in which l /6 is light and % 
is dark, is the same as one in which % is light and Yt> is dark 
( SQ2a). 

592a. A formula to express these factors numerically was worked out by 
T. C. Porter of Eton College and published in the Proceedings of the Royal 
Society,, Vol. 63, p. 347; Vol. 70, p. 313-329 (1902). 

The constants have been recalculated. 

Let f = number of light flashes per second at which flicker disappears when 
light and dark flashes are equal. 

Let n = number of flashes per second; light and dark flashes are unequal. 

w = angle of white space in disc. 

(360 w) = angle of dark space in disc. 

I = intensity of illumination in meter candles. 

b = constant depending on illumination. 

From experimental data the formula comes out 

f = 26 -f- 12.2 log I 

b = 12.04 + 2 -3?8 log I 

n = f + b [logw log (360 w) 4.5106]. 



CH. XI] PRECAUTIONS FOR MOVING PICTURES 427 

592b. Table showing Speed at which flicker just disappears. 
FROM T. C. PORTER 

Flashes per second 

Illumination Logarithm of at which flicker 

meter candles illumination just disappears 

.0625 8.796-10 17.75 

.in 9.046-10 18.08 

.25 9.398-10 18.50 

i.oo o.ooo 25.08 

4.00 0.602 33.50 

1.56 0.193 28.00 

2.70 0.431 32.00 

6.30 0.799 35-50 

25.00 1-398 42.66 

IOO.OO 2.OOO 5O.I6 

IOO.OO 2.OOO 5O.83 

178.00 2.250 55.08 

400.00 2.602 56.42 

1600.00 3.204 65.00 

6400.00 3.806 71.00 

RESULTS FOUND BY THE AUTHORS WITH A MOVING" PICTURE OUTFIT 
32.00 1.500 36 

IOO.OO 2.OOO 41 

1000.00 3.000 50 

3200.00 3.500 54 

The curves (fig. 234) are drawn to show the speed at which 
flicker disappears for equal light and dark flashes. There is not a 
great advantage as far as the speed at which flicker disappears in 
having the duration of the dark flash very short. The actual 
appearance of flicker is much less violent, however, when the dark 
section is narrow. 

GENERAL PRECAUTIONS 

593. Inspection of films. Before attempting to show films 
to an audience, it is well to inspect them carefully to see that they 
are in good condition and wound on the reel correctly. 

Use the rewinder to roll the film from the new reel upon an empty 
reel. Turn the handle slowly with one hand while holding the 
edge of the film between the fingers of the other hand; do not touch 
the face of the film. When a patch is met in the film inspect it 
carefully to see that: (i) The same side of the film is on top. (2) 
The patch is made at the right place so there will not be a misframe, 



428 PRECAUTIONS FOR MOVING PICTURES [Cn. XI 

i. e., see that the pictures are evenly spaced. (3) The sprocket 
holes match evenly. (4) That the patch is strong and no loose 
corners stick up. 

If the patch is not good in all these particulars, it must be 
remedied. 

There should be no torn sprocket holes or torn places in the film 
or bad scratches in the emulsion. If any such defects are found, 
they should be cut out and the film patched together again. Places 
may be found where the film broke and was pinned together. 
Remove the pin and cement the film. 

When the whole film has been inspected in this way, rewind it, 
so that it will go through the machine correctly. 

See that there is a "leader" or strip of blank film i to 2 meters 
(4 to 5 ft.) long to thread through the machine, so the entire title 
of the film may be shown. The part of the film used to thread the 
machine often becomes broken and a good "leader" saves the film 
itself from damage. 

If there is time, it is well to run the film through the machine and 
watch the screen picture before showing it to an audience. 

594. Splicing the film. When moving pictures are to be 
shown the operator will need to patch the film occasionally. Often 
a film breaks or an old splice comes in two. A splice is made 
by cementing the two ends of the film with "Film Cement." 

Cut one end of the film, b, (fig. 235), exactly on the line between 
two pictures and scrape the back (shiny side) of the film with a 
sharp knife. There may be oil on the film. It must be removed; 
cement will not hold otherwise. Cut the other end of the film a, 
about 4 mm. (}/% in.) longer than a dividing line between two pic- 
tures. Then scrape off the emulsion between the picture division 
and the ends of the film. This emulsion can be scraped off 
accurately to the line by holding a straight edge over the picture 
on a, and letting the end of the film project. Scrape the emulsion 
off and right down into the film stock. Scrape the corners as well 
as the middle, as the corners usually are the first to work loose. 
Film cement is then spread on the back of b, and the front of a, 
with a brush or stick, never use the fingers. Be sure to get plenty 



CH. XI] 



PRECAUTIONS FOR MOVING PICTURES 



429 



of cement on the corners of the 
film. Then immediately press 
the two ends of the film together 
firmly for a few seconds, being 
careful not to push the ends of 
the film sidewise in doing so. 

Several points must be care- 
fully observed in order to get a 
splice which is satisfactory and 
durable. 

i . Cut the film so that the di- 
viding lines between two pictures 
come exactly together or there 
will be a "misframe" when the 
film is running through the ma- 
chine. 




FIG. 235A. EDISON FILM MENDER. 

(Cut loaned by the Edison Manu- 
facturing Company). 

It has three gates or hinges those 
on the sides clamp down and hold 
the film while the ends are cut and 
prepared and the cement is applied. 
The narrow middle clamp is then 
closed holding the ends of the film 
firmly in contact while the cement 
dries. The gauge shown at the left 
enables the operator to cut true edges 
on the film and scrape the proper 
width for the cementing. 




FIG. 235. METHOD OF PATCHING A 
MOVING PICTURE FILM. 

One end of the film B, b is cut on 
the line between two pictures and 
the other end A , a, is cut a short dis- 
tance beyond the line between two 
pictures. The film side of one and 
the shiny side of the other are 
scraped, cement is applied and the 
two ends are placed together so 
that the sprocket holes will match. 



2. Scrape the film well, 
both the back side of b, and 
the emulsion side of a. 

3. Apply the cement and 
work rapidly. 

4. Be sure to hold the 
emulsion side of both films 
either up or down. 



430 PRECAUTIONS FOR MOVING PICTURES [Cn. XI 

5. Get the film together so that the two parts of the film are 
in a straight line and not at an angle. 

6. Get the sprocket holes together, so that they will match 
accurately. 

7. Press the film firmly together without any sidewise motion. 
It is well to practice on short pieces of scrap film until strong 

splices fitting together accurately can be made quickly. 

There are two kinds of film cement, one which is good for cellu- 
loid films only, the other (NI cement) will work equally well on 
non-inflammable film and celluloid film. 

For making permanent patches in a routine way there is a film 
mender (fig. 23 5 A), consisting of a guide and a pressure clamp, so 
that the film maybe accurately held while being cemented together. 

All splices should be as far as possible made before beginning a 
performance. Any old splices which appear weak and likely to pull 
apart should be pulled apart and cemented together again. 

With the greatest precaution a film will sometimes come apart 
during an exhibition. When this occurs the film is pinned together 
to be spliced permanently later. Be sure to remove pins and make 
permanent splices before attempting to run the film through the 
machine again. 

WINDING AND REWINDING 

595. A device to wind the film from one reel to another is a 
part of any moving picture outfit. 

While passing through the machine the film is always wound on 
the lower reel in the wrong direction for use, and it is necessary to 
rewind it, so that it will be right side out again. 

While rewinding is the time to remove pins and splice per- 
manently any breaks in the film which occurred during an exhibi- 
tion. 

In most moving picture theaters one film is rewound while the 
next film is being shown, the operator turning the moving picture 
crank with one hand and the rewinder with the other hand. When 
the rewinding is done this way very rapidly and the rewinder is 
fastened to the walls of a sheet iron booth a decidedly terrifying 
sound may be produced. 



CH. XII DANGER OF FIRE 431 

DANGER OF FIRE 

596. Before the introduction of non-inflammable films, all 
films were made by coating the emulsion upon celluloid. This is a 
nitrate (the trinitrate) of cellulose to which is added a certain 
amount of camphor. A more highly nitrated cellulose is called gun 
cotton. 

There is sufficient oxygen in the nitrated cellulose to partially 
support combustion and it is the cause of the highly inflammable 
nature of celluloid. This was strikingly shown in some experi- 
ments made to ascertain the possible danger from an ignited film. 
A small reel of film was lighted and put under a tin box so that no 
air could get at it. A fire in ordinary combustibles, such as paper 
or wood, would soon be smothered, but the roll of film continued 
to decompose in the closed box. This shows that if a roll of film, 
even in a closed fire proof magazine, once catches fire it will con- 
tinue to burn as long as there is anything left of it. 

The gases given off from the film decomposing in a closed box are 
very disagreeable and will burn in contact with air if they are once 
lighted. If celluloid will burn so vigorously in a closed box, what 
would be the effect of a large reel of film lying uncoiled in a waste 
basket or on the floor should it once catch fire ? This was the prac- 
tise in the early days of the art of projecting moving pictures. 
Seven to ten meters (twenty or thirty feet) of film piled loosely, will 
be completely consumed in a few seconds, burning with a fierce 
flair e while it lasts. 

In view of this very evident danger, modern, apparatus is 
designed to make it as safe as possible. To the good design of the 
machine must be added the cooperation of the operator to prevent 
a fire. 

The fire shutter (fig. 228), automatically closes whenever the 
machine is not running. This shutter is placed in front of the film 
and prevents the light of the arc from striking it except when it 
is in motion. If the film should break, however, a tag end might 
remain in the aperture plate and be ignited, the fire shutter remain- 
ing open while the crank was being turned. To prevent this 
trouble the light should be instantly shut off whenever a film 
breaks. 



432 



DANGER OF FIRE 



[CH. XI 



The time required for igniting a film was examined. It was 
found that an ordinary film, partly black and partly transparent 
when held in the condenser focus would first curl and later burst 
into flame. The time required for each was noted, first with, then 
without a water-cell. 



Image of arc 



No water-cell 
Curl Burn 



With water-cell 
Curl Burn 



20 Ampere D. C. Arc 

Concentrated spot 1.3 sec. 

vSmall spot 2 sec. 

24 Ampere A. C. 

Concentrated spot 6 sec. 

35 Ampere A. C. 

Spot large enough to project 

picture, film dead black ... 3 sec. 



2.6 sec. 
3. 5 sec. 



5 sec. 
7 sec. 



losec. 
12 sec. 



losec. over 30 sec. 



1 2 sec. over 60 sec. 



With 3 5 amperes alternating current and the crater image large 
enough to project the full size of picture, the film curled in 3 seconds 
and burst into flame in 12 seconds. When a water-cell was used 
the film was merely slightly warped and not in the least injured 
after an indefinite exposure. With larger installations the water- 
cell could not be relied on to protect the film indefinitely, though it 
would much reduce the risk. 

The data given in 848 (fig. 342), shows the effects of the 
water-cell in reducing the radiant energy. 

Examination was made of the probable security afforded by the 
fire-trap of a fire-proof film magazine. A short piece of film was put 
through the fire-trap of a film magazine. This fire-trap consists in 
a flat tube, the lower end of which is nearly closed by a pair of 
metal rollers. The flame would not follow the film through the 
metal tube. When, however, the film was pulled rapidly through 
the fire-trap it might or might not be extinguished by the rollers. 

With the upper magazine, where the film hangs down, the rising 
flames heated the film to such an extent that when pulled upward 
through the fire-trap it continued to burn on the other side. When 
the film projecting from the lower magazine was ignited and pulled 
down through the fire-trap, it was extinguished just as a strip of 



CH. XI] MOVING PICTURE EXHIBITION 433 

paper would be. The end of the film did not get as hot as that 
projecting from the upper magazine because the rising flames did 
not tend to play around the unburned part. It would seem, there- 
fore, that the fire would probably not be carried into the lower 
magazine along with the film. Of course, with the upper magazine 
the film is going out of the opening in normal operation. What 
would be the effect of the sharp blaze from a meter or more (three 
feet) of loose film which would quickly unwind if the film broke can 
only be conjectured. It would be likely to get the magazine red 
hot and set the film inside on fire. 

With these possibilities of risk in mind, one will naturally be very 
careful in handling the apparatus, so that nothing shall start to 
burn and to follow the precautions of keeping all of the films not 
in use inside of fire-proof boxes. The two films in use are : the film 
in the machine, and the film which has just been run through and is 
being rewound. 

When non-inflammable film is used the above precautions are not 
necessary from the standpoint of fire risk, but the films might be 
spoiled. It is, however, a good plan to be careful even if non- 
inflammable films are used, so that habits of carelessness will not 
lead to accident should one of the celluloid films be included with- 
out the knowledge of its nature. 

THE CONDUCT OF AN EXHIBITION 

597. Inspection of the plant. Is the exhibition going to go 
smoothly, without hitches, or will the light be poor and go out, the 
film be out of focus, and break and everything go wrong? This 
depends largely upon the operator and a very careful inspection of 
all the apparatus before the exhibition begins. 

The principle things to look out for are: 

(1) See that all wiring is in good shape, no binding posts loose, 
no wires almost burned out in the lamp. 

(2) See that the carbons in the lamp are long enough, that extra 
carbons are ready, that tools to change carbons are handy. 

(3) Burn the arc a little while till the carbon ends are properly 
shaped. 



434 MOVING PICTURE EXHIBITION [Cn. XI 

(4) See that the optical parts are clean, and free from dust. 
See that everything is in line and the light is even on the screen. 

(5) See that the objective is in focus. 

(6) See that the mechanism is oiled and in good order, no screws 
loose. 

(7) See that the films are properly mended and that there are 
no misframes. 

(8) See that the rolls of film are in the proper order. 

The first reel of film is put in the magazine and the machine is 
threaded. 

The arc is either pushed away from the operator so it will not 
shine on the moving picture head or else the dowser in front of the 
condenser is let down. 

The arc lamp is lighted. When all is ready the crank of the 
machine is started, the arc lamp pulled toward the operator into 
position, the dowser is raised, and the house lights turned off 
all at the same time. 

During the exhibition there should be but two things to watch. 

1. The adjustment of the carbons. The carbons need occa- 
sional attention to keep a good light. 

2. The action on the screen. The action on the screen should 
be very carefully followed. It will serve as a guide to the proper 
speed to turn the crank of the machine. The lighting of the picture 
and the focus of the objective may need attention occasionally as 
can be seen by watching the screen. 

If the machine or the film is poor various mishaps may occur and 
require a short stop. 

The most frequent is a misframe. This occurs when a .patch 
has not been properly made and the pictures not properly matched. 
The difficulty is remedied by raising or lowering the framing lever. 
Note the place where the misframe occurs and remove it before the 
film is shown again. , 

The film may, break. Turn off the light instantly, or push the 
lamp over to the lantern-slide side or lower the dowser. If a tag 
end of film is left in the aperture plate, it may catch fire if the light 
is not turned off. The film is now threaded through the machine 



CH. XI] MOVING PICTURE EXHIBITION 435 

again and the ends pinned together in the lower film magazine. 
Splice permanently later. 

When the end of the film is reached, turn up the house lights and 
put out the arc light, or push the lamp over to the lantern-slide side 
as the case may require. Turn the crank a few times until the film 
has all rolled into the lower film magazine. 

The lower reel is taken out and put on the rewinder, the empty 
reel from the upper magazine put in its place and a new roll of film 
is put in the upper magazine. 




FIG. 236. THE EDISON HOME KINETOSCOPE. 
(Cut loaned by Thomas A. Edison, Inc.). 

At the end of the exhibition all of the films are rewound and put 
in the box to be kept until the next day or to be sent away. 

598. Home projectors and advertising magic lanterns. In 

addition to the regular moving picture machines there have been 
two side-line developments. One of these is a relatively cheap 
moving picture machine with a small arc lamp for the house light- 
ing system ( 127) or some other form of radiant (Ch. IV, V). 
Some of these small instruments like the "Phantoscope" of Jenkins, 
take the standard size of motion picture film. Edison has put out 
another form, the "Home Kinetoscope," (fig. 236). This does not 
project the ordinary size of moving picture, but very small pic- 
tures. Instead of one row of pictures on the film there are three 
rows. With the small pictures in three rows, a film 80 feet (24.38 
meters) long contains as many pictures as 1000 feet (304.8 meters) 
of the ordinary moving picture film, and the mechanism is so. 
arranged that the three rows are shown without a break. 



436 TROUBLES WITH MOVING PICTURES [Cn. XI 

The automatic magic lanterns are devised to show automatically 
a series of ordinary lantern slides. One of these instruments is 
called the "Advertigraph" by Williams, Brown & Earle and has a 
capacity of 24 lantern slides. Another form, designated a 
"Stereomotorgraph" by the Charles Besler Co., has a capacity 
of 52 lantern slides. These instruments are very effective for 
advertising and for exhibitions in museums. 

TROUBLES 

599. There are two main troubles confronting the moving 
picture operator: A poor screen image, and fire in the operating 
room. 

A poor screen image. This may be due to any one or a combina- 
tion of the following: 

(1) An operator with insufficient knowledge and experience. 
This is probably the most common cause. 

( 2 ) A poor pro j ection apparatus . 

(3) A bad light due to insufficient current or to a wrong relative 
position of the carbons. 

(4) The parts of the projection apparatus not on one axis. 

(5) The film may be poor; too dark or not sharp, or worn out, 
or badly perforated, or scratched, giving rainstorm appearances. 

(6) The film may be wrong side up or wrong side out in the 
machine. 

(7) There may be a "misframe" ( 584, 597). 

(8) The apparatus or the floor may vibrate, giving a jerky 
appearance on the screen. 

(9) The shutter may not be in the right position or of the right 
design, hence flicker, travel ghost, etc. 

(10) The general light in the room may be too great, hence, a 
gray picture without sufficient contrast. The same effect is pro- 
duced by a single room light or the light from a door or window 
shining directly on the screen. 

Fire in the operating room. This seems inexcusable, but may 
occur. To avoid loss of life and of property the operating room 
must be (i) truly fire-proof; (2) it must have a large flue leading 



CH. XI] TROUBLES WITH MOVING PICTURES 437 

to the open air outside the building; (3) all the openings in the 
operating room must be closed by fire-proof shutters the instant a 
fire starts. In this way the smoke and gases will escape through 
the flue, and no one in the audience will know that anything is 
wrong. 

From the standpoint of the operator, if a fire should start he 
should turn off the arc light and turn on the room lights as soon as 
possible. If there is a pail of water or a small fire extinguisher 
of the wet form in the room the water or the fire extinguisher can 
be used to good advantage to prevent the fire from spreading. 
The cooling effect will sometimes put out the film, although, as 
stated above exclusion of oxygen does no good for the celluloid 
contains enough oxygen to support combustion. The real way 
after all is to be so careful that a fire never starts. (See Richard- 
son's Handbook, zd edition, pp. 65-93). 



438 



DO AND DO NOT WITH MOVING PICTURES [Cn. XI 



599 1 . Summary of Chapter XI: 



Do 

1. Learn the principles, and 
perfect yourself in the practice 
under expert guidance, before 
you assume the responsibility of 
an independent operator. 

2 . Keep your operating room 
in perfect order. 

3. Light the theater so that 
the lights cannot shine directly 
in the eyes of the spectators or 
upon the screen. 

4. Have a perfect screen. If 
it is a painted screen, add a fresh 
coat occasionally. 

5. Use direct current for the 
arc lamp if possible (Ch. XIII). 

6. Inspect wiring and appara- 
tus daily. 

7. Keep the lenses of the con- 
denser and of the objective 
clean, and in the right relative 
position. 

8. Keep in mind the precau- 
tions ( 593-594). 

9. Leairn to conduct the ex- 
hibition in the best possible 
manner. 

10. Remember that it is far 
easier to avoid a fire than to put 
it out. 



Do NOT 



1. Do not pretend to be a 
competent operator until you 
have the requisite knowledge 
and experience, and then never 
stop learning. 

2. Do not have your operat- 
ing room in disorder. 

3. Do not install room lights 
so that they can glare in the 
eyes of the spectators or shine 
on the screen. 

4. Do not project on a dirty 
screen. 

5. Do not use alternating 
current for projection if you 
can use direct. 

6. Do not neglect a careful 
daily inspection of wiring and 
apparatus. 

7. Do not use dirty lenses or 
objectives. 



8. Do not fail to study care- 
fully the precautions ( 593). 

9. Do not neglect the direc- 
tions for the conduct of an 
exhibition. 

10. Never forget the danger 
from fire. 



CHAPTER XII 
PROJECTION ROOMS AND SCREENS 

600. Apparatus and Materials for Chapter XII: 

1. Room which can be made entirely dark, or which can be 
partly lighted, depending on the kind of projection and the 
radiant. 

2 . If for exhibitions, the room should have plenty of aisles and 
exits, and there should always be lights (red lights) near the exits, 
and these lights should be independent of the projection circuit. 
The room should be well ventilated, and of a form found suitable 
for audiences, e. g., like a church, theater or university lecture 
room. The room should be tinted and decorated with light-absorb- 
ing colors ( 604). 

3. The lantern or other projection apparatus should be so 
placed that it does not interfere with the audience ( 612-620). 

4. Special room for the projection apparatus. If in a moving 
picture theater, there should be a fire-proof room for the apparatus. 
This should have a large ventilator extending through the roof or 
side of the building ( 556-5 57). 

5. Screen upon which the images are projected. This should 
receive the image at right angles to avoid distortion (fig. 241), and 
be of sufficient size for the room ( 633). 

601. For the historical consideration of rooms and screens 
see under history in the Appendix. See also the works referred to 
in Chapter I, 2, and the catalogues of manufacturers of projection 
apparatus and materials. Periodicals on moving pictures like the 
Moving Picture World; F. H. Richardson's Motion Picture Hand- 
book; and F. A. Talbot's Moving Pictures. 

602. Suitable room for projection. Any room which can be 
darkened may be used for projection, but to be satisfactory it 
should have the qualities of a good auditorium. 

(1) There should be plenty of aisles and passages, so that the 
auditors can easily reach their seats. 

(2) There should be plenty of exits, so that the room can be 
quickly and safely emptied. 

(3) There should be plenty of fresh air. 

439 



440 PROJECTION ROOM [Cn. XII 

(4) Each seat should have a good view of the stage and the 
screen. 

(5) There should be enough diffuse light in the room so that 
people can find their way around easily and after gaining twilight 
vision, be able to take notes. 

603. Form of the room. In general that shape of room which 
has been found most satisfactory for churches and theaters and for 
science lecture rooms in colleges and universities is well adapted for 
projection. As, however, the entire attention must be given to 
the images on the screen in the middle of the stage there is a ten- 
dency to make the rooms used especially for projection longer than 
they are wide. In a room which is approximately square, the 
spectators who sit at the sides of the room near the front do not 
have so good a view of the screen as those in the middle of the room 
and farther back. 

With a long narrow room either the picture must be magnified 
excessively to enable those on the back seats to see the details, 
while for those on the front seats the pictures seem very coarse, or 
there must be a compromise so that only for those in the middle of 
the hall are the screen pictures of the most favorable size. 

We strongly advise any person having the responsibility of 
planning a lecture hall for educational purposes or for exhibitions, 
to take advantage of human experience and see a considerable 
number of halls in various places, and get hints of what not to do 
as well as of what to do from those who have had experience. 
Then he can combine excellencies and avoid mistakes in planning 
his own building or room. 

604. Tint and decoration of the room. In order to get the 
best possible results in projection, no light whatsoever should reach 
the eyes of the spectators except that reflected from the screen. 
With the moderate light available for the earliest users of the magic 
lantern it was advised that the walls and ceiling be made black so 
that, as they put it, "the room would be as sombre as possible." 
For some experiments in projection with polarized light, the 
spectroscope, and the highest power micro-projection such a room 



CH. XII] LIGHTING THE PROJECTION ROOM 441 

would still be an advantage; but for ordinary magic lantern and 
moving picture exhibitions total darkening of the room is unneces- 
sary and undesirable. But for all projection it is a great advan- 
tage to prevent any light from falling upon the screen except that 
from the projection apparatus. The room should therefore be 
tinted with some light-absorbing color. Nothing is better than the 
brownish color of natural wood, such as oak or pine. If natural 
wood is not used, the walls and ceilings can be tinted brownish 
or olive. For decorations, rich, dark red, orange, green, and blue 
may be used. Light orange, green, and blue reflect too much light 
but the dark, rich colors give the pleasing effect without making 
the room too light. 

For mixing these tints, if oil colors are used, much turpentine 
should be employed to give a flat or dull finish, not a shiny or glossy 
one. If the finish is shiny it will act like a mirror and give an 
undesirable glare, and shine in the face of some of the auditors. 

605. Light in the exhibition room. For magic lantern and 
moving picture exhibitions, the room should be light enough so 
that the spectators can easily find their way about ; and after the 
twilight vision is established, the spectators should be able to take 
notes easily. 

If the room is finished and decorated with light-absorbing colors 
and tints as indicated above, there is no danger of making the 
screen images gray and dull from reflections from the walls and 
ceiling. One has simply to guard against direct light shining on the 
screen from a window or from a lamp. (For lighting a black-board 
in a lecture room see fig. 240). 

606. Lamps for general lighting. The lamps to give the 
needed light should be so arranged, and with such shades that: 

(1) They cannot shine directly in the eyes of the spectators; and 

(2) That they cannot send any of their rays directly upon the 
screen. This is best accomplished by placing the lights along the 
sides of the room or on the ceiling or both, and shading them so that 
none of their light can extend directly to the screen. 

The arrangement sometimes used of a row of lights around the 
screen is bad; for, while no light can reach the screen from them, 



442 



LIGHTING THE PROJECTION ROOM 



[On. 



the glare in the eyes of the spectators will detract from the effect. 

If ceiling lights are used they should be placed close to the ceiling 
and on the side of the construction work (stringers, etc.) away from 
the screen. Then the light will extend obliquely downward and 
backward, but none of it will fall directly upon the screen. 

Lights along the sides of the room can be placed behind the 
projecting construction work, or shaded so that the light cannot 
extend toward the screen. 




FIG. 237. METHODS OF INDIRECT LIGHTING. 

(Cuts loaned, joy the National X-Ray Reflector Co.). 

A Shows an opaque bowl containing the electric light. The light is 
reflected upward and is diffused throughout the room. 

B and C Illustrate the indirect lighting where the bowl containing the 
electric light allows a certain amount of the light to extend downward. The 
light is also reflected upward as in A. 

In C the bowl is cut away to show the electric bulb, the reflector for throwing 
the light upward, and the opal glass diffuser below to give the soft luminous 
effect of a very large source in the "luminous bowl." 



CH. XII] DARKENING THE PROJECTION ROOM 443 

The indirect or concealed light sources which have been recently 
developed answer all the requirements for suitably lighting a mov- 
ing picture theater or, indeed, any other place where a soft light is 
required and the light should not shine directly in the eyes of the 
spectators (fig. 237 A, B, C). 

It is also an advantage to have the screen in a kind of alcove i to 
2 meters (3-6 ft.) deep and the walls on the sides, the floor and the 
ceiling dark brown or dark red or olive to absorb any light reflected 
upon 'them (6o6a). 

For exhibitions, it also adds brilliancy to the picture to have a 
black border around the screen. It gives also the effect of a framed 
picture. 

With such an arrangement of the lights in a suitably tinted room, 
no light will reach the screen directly to destroy the contrast and 
render the image vague. There can be sufficient diffused light in 
the room to enable one on entering to see the aisles and seats, and 
go about without stumbling. In a short time twilight vision will 
be established and it will then be possible to read or to take notes. 

607. Red lights near all exits. Fire escapes. In public 
halls, and especially in moving picture theaters, it is an advantage, 
and often a requirement in city regulations, to have red lights near 
every exit so that the audience can see exactly where it is possible 
to get out of the hall. 

The manager of every public hall should look to it every day that 
the fire escapes are in working order and before every exhibition 
that the doors or gates to the fire escapes are unlocked and easily 
opened. 

608. Relative darkness of the room for different kinds of 
projection. The amount of diffuse light permissible in the pro- 

606a. While it is a great help to have a screen in a dark alcove, still the 
general light of the room, although none extends directly upon the screen, 
tends, if too great, to make the image less brilliant and definite. Every one 
who has studied astronomy at all with a telescope knows full well how the 
defmiteness of the image of a nebula or dim star cluster diminishes when the 
moon rises and floods the heavens with its diffuse light. One can also see the 
effect of too much diffused light by observing a lighted clock face on a dark 
night, and the same face with the same light shining from it on a moonlight 
night or early in the evening twilight before complete darkness. 



444 DARKENING THE PROJECTION ROOM [Cn. XII 

jection room depends entirely upon the brilliance of the screen 
image. In order to see the screen image clearly there must be 
strong contrast between it and surrounding objects. With trans- 
parent lantern slides and sunlight or the electric light to illuminate 
them one can see the screen images well in a room so light that 
everything in the room is visible provided no direct light reaches 
the screen except 'that from the projection apparatus. If the 
lantern slides are less transparent or the light used for projection 
less brilliant, then the room must be relatively darkened to give 
the needed contrast. Keeping the principle of contrast in mind, 
one readily understands that for some of the experiments in physics 
where the light on the screen is very dim, with kinemacolor moving 
pictures and with Lumiere colored lantern slides, and with high 
power micro-projection, the room must be very dark in order to get 
the screen image clearly visible. In like manner if the source of 
light for projection is relatively weak, like the acetylene flame or 
some other less brilliant light than the electric arc, the room must 
be darker than with a more brilliant radiant. 

609. Daylight and twilight vision. It has been known for 
time out of mind that with most people the eyes can adapt them- 
selves to a dim light or to a bright light. If one goes into a dimly 
lighted room from full daylight the room will at first appear per- 
fectly black, but in a few minutes objects can be seen fairly well, 
and within half an hour the room will appear comparatively light. 
On the other hand, in passing from a comparatively dark room to 
full sunlight the eyes are so dazzled at first that hardly anything 
can be seen, but soon the eyes become adapted to the bright light. 
It has been found by careful experiments on large numbers of 
people that the main adaptation of the eyes for bright light after 
being in a dark room requires only about 6 minutes, while the 
adaptation for a dim light after being in full daylight requires 
about 30 minutes, although after 10 minutes the eye is about 100 
times as sensitive in a dark room as it is in full daylight. While 
the pupil expends normally in dim light, thus increasing the aper- 
ture of the eye, this is not the fundamental thing in adaptation, but 
there is some change in the retina which gives it greater sensitive- 
ness. 



CH. XII] DARKENING THE PROJECTION R 

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FIG. 238. FACE AND SECTIONAL VIEW OF WINDOW SHADES PLANNED FOR 
IN THE CONSTRUCTION OF THE BUILDING. 

A Cross section showing the window shade (Sh) in the grooves (W W) at 
the sides. 

B Face view of the window with the shade (Sh) shown by dotted lines. 

C Sectional view of the window showing the window sashes (W S), the 
ordinary window curtains (Cr) close to the sash, and the window shade (Sh) 
considerably in front of the curtain, i. e., near the front of the window frame. 

The coping over the window is shown by dotted lines as turned down. 
This exposes the shade roller so that it can be adjusted if it gets out of order. 

The window shade is shown as drawn down, and the shade string goes over a 
pulley (P) and is caught in a fork-like holder (C) in front of the window frame. 



446 DARKENING THE PROJECTION ROOM [Cn. XII 

Much of the projection at the present time requires daylight 
rather than twilight vision from the brilliancy of the screen images, 
but one should keep in mind that good screen images may be 
obtained by two methods (i) brilliant illumination and daylight 
vision; or (2) moderate illumination and twilight vision. 

610. Method of darkening a room. As many rooms used for 
projection are well supplied with 'windows there must be some 
method of excluding daylight or other outside light. The two 
means usually employed are wood or metal shutters and opaque 
cloth curtains. 

Shutters may be on hinges and swing sidewise, or they may be 
hung, and by means of pulleys raised and lowered. In many 
laboratories where the shutters are opened and closed several times 
during a lecture, there is a water or electric motor to move the 
shutters. 

If curtains are used they should be of dark colored opaque cloth 
on a spring roller, so that they can be opened or closed as much or as 
little as desired. These are usually opened and closed by hand 
(fig. 238). 

611. Excluding light at the window margins. As curtains 
are usually hung, there is a space admitting light at the top, bot- 
tom, and sides of the window. This can be avoided by having the 
edges of the curtain in a groove at the sides and bottom of the 
window frame, and having the curtain roller above the opening of 
the window frame (fig. 238). If one has the designing of the 
building, proper grooves can be planned for and put in when the 
window frames are made. If this has not been planned for in 
designing the building, then the light-excluding devices can be 
added afterwards. That is, a light-excluding shield can be put all 
around the window frame (fig. 239). This will, of course, cut down 
somewhat the opening of the window frame. 

POSITION OF THE PROJECTION APPARATUS IN THE ROOM 
612. The best position for the projection apparatus in a lec- 
ture room or exhibition room is at the back of the room, where it is 
entirely free from the audience. This also gives the operator 
greater freedom (fig. 240). 



CH. XII] 



DARKENING THE PROJECTION ROOM 



447 



Sh 

B 



Sh 




FIG. 239. FACE AND SECTIONAL VIEW OF A WINDOW SHOWING HOW THE 

LIGHT-EXCLUDING SHADE CAN BE INSTALLED AFTER THE BUILDING is 

CONSTRUCTED. 



448 



POSITION OF PROJECTION APPARATUS [Cn. XII 



A Cross section showing the window shade (Sh) behind the thin boards 
(W W) which serve to exclude the light at the top, sides and bottom of the 
shade. 

B Face view of the window with the" light-excluding shade (Sh) shown in 
dotted lines, (L) indicates the size of the window frame. The sash cuts this 
down somewhat and the thin board frame to cut out the light around the edge 
of the curtain cuts it down considerably more. 

C Lateral view of the window with the shade in dotted lines. The light- 
excluding frame around the edge is in full lines in B and C. 

613. Position of the projection apparatus with a level room. 

In a level room, the projection apparatus at the back of the room 
must be at such a level that the projection beam goes over the 
heads of the spectators. This can be accomplished by building a 
platform, or by using a high table. In case the image is still not 
high enough on the screen, the lantern can be tilted slightly upward 
by putting a wedge under the end of the baseboard supporting it 
(fig. 240).. 




FIG. 240. SECTIONAL VIEW OF A LECTURE ROOM HAVING A GALLERY. 

B Black-board. This is lighted by incandescent lamps behind a curved, 
metal shield (H L). This gives plenty of light for the black-board without in 
any way injuring the brilliancy of the screen image. 

L T Lecturer's table on the platform (P). 

Ml The magic lantern in the gallery on its table and special support ( T). 

Sc Screenjfor the image above the black-board. 



CH. XII] 



POSITION OF PROJECTION APPARATUS 



449 



614. Level room with the apparatus near the screen. It is 

sometimes desirable to put the apparatus near the screen. Then 
provision must be made by removing some of the seats if the center 
aisle is not wide enough. 

The apparatus must usually be raised somewhat also, and some- 
times the objective inclined more or less upward. In case it is 
desired to have the apparatus very near the screen it must be 
pointed upward considerably and then the screen should be hinged 
at the bottom so that it can be inclined toward the lantern till it is 
perpendicular to the optic axis. The simplest way to fix the 
screen in any position, and to change the position is by means of 
ropes and pulleys at the top. 




FIG. 241. 



LECTURE ROOM WITH RISING SEATS, AND THE LANTERN IN THE 
MIDDLE OF THE ROOM, NOT AT THE BACK. 



B Black-board lighted by the hidden lights (H L) behind a curved metal 
shield. 

L T Lecturer's table in front of the audience. 

Ml E The magic lantern (Ml) ; its rays shown in full lines, and the episcope 
or opaque lantern (E) with its rays shown extending from the mirror (M) in 
dotted lines. 

Sc The screen for receiving the image. As the magic lantern must be 
elevated the screen is tipped toward it to meet the axial ray at right angles. 

For such a position of the magic lantern the projection objective must be of 
shorter focus to give the desired size of image than when the lantern is at the 
back of the room ( 636). 



450 POSITION OF PROJECTION APPARATUS [Cn. XII 

The lantern should be fastened to a hinged board when it is 
elevated considerably (fig. 118, 242). 

615. Magic lantern on the lecture table. Occasionally it is 
an advantage to have the magic lantern on the lecture table ; then 
the lecturer can manipulate it himself. 

There are three arrangements possible: (i) The lantern is 
pointed toward a screen at the side of the room (fig. 243). (2) It 
is pointed obliquely upward toward the screen in front of the 
audience. In this case the screen must be inclined toward the 
lantern as indicated above (614). (3) Occasionally, for ease of 
manipulation, the lantern is pointed obliquely upward toward the 
audience and a plane mirror reflects the image-forming rays back- 
ward to the screen (fig. 244). If a mirror is used, the lantern slides 
must be inserted with their faces toward the objective. 




FIG. 242. MAGIC LANTERN TABLE WITH HINGED BASEBOARD. 
Hinges connect the baseboard to the table at the left. By putting a block 
under the board at the right, it can be elevated to bring the screen picture 
higher up (fig. 118). 



CH. XII] POSITION OF PROJECTION APPARATUS 



451 



616. Projection with inclined seats or gallery. If the seats 
in the auditorium are raised after the manner of an amphitheater or 
if a gallery is present, in many cases the apparatus can go to the 
back of the room or in the gallery. This may make it necessary to 
point the projection apparatus somewhat downward towards the 




FIG. 243. GROUND PLAN OF A LECTURE ROOM WITH THE MAGIC LANTERN 

ON THE LECTURER'S TABLE AND THE SCREEN AT THE SIDE OF THE ROOM. 

B Black-board. 

Ml Magic lantern on the lecture table (L T) and pointing up to the screen 
(Sc) on the side of the room. 

Sc The screen is shown tipped forward to avoid distortion. 

Such a position of the lantern enables the lecturer to perform experiments or 
show lantern slides conveniently. 



452 



POSITION OF PROJECTION APPARATUS [Cn. XII 



screen, but as the distance is usually considerable, the screen image 
will be good on a vertical screen. The position of the lantern 
should never be so high that the screen image will be distorted. 

617. Apparatus in the middle of the auditorium with raised 
seats. If the apparatus cannot be at the back of the room in an 
amphitheater then a space or alcove must be made somewhere in 
the middle by omitting a certain number of seats. The machine 
is liable to be more or less distracting if in the middle of the room, 
but sometimes this cannot be avoided on account of distance or the 
form of the amphitheater (fig. 241). 




FIG. 244. PART OF A LECTURE ROOM WITH THE MAGIC LANTERN ON THE 

LECTURE TABLE DIRECTED TOWARD THE AUDIENCE AND A MIRROR TO 

REFLECT THE IMAGE ON THE SCREEN IN FRONT OF THE AUDIENCE. 

B Black-board with hidden light behind the curved metal shield (H L). 

Ml M Magic lantern pointing toward the audience. The mirror reflects 
the image back to the screen in front of the audience. The mirror also serves 
as a shield. 

Sc The image screen. By means of the pulley and cord it is inclined on its 
hinges at the lower edge toward the mirror of the magic lantern. In this case 
it is not inclined sufficiently to meet the axial ray at right angles, hence there 
will be some distortion of the image and the upper edge will not be in sharp 
focus when the lower edge is. 

T Lecturer's table. With such an arrangement the lecturer can demon- 
strate with the lantern conveniently, and still have the screen in front of the 
audience. If he uses lantern slides they must be put in the holder facing the 
objective, not the light or there would be a mirror image on the screen (fig. 213). 



CH. XII] 



POSITION OF PROJECTION APPARATUS 



453 



Occasionally when the seats are on a steep incline there is left a 
space through which the projection objective can send its beam to 
the curtain, the apparatus and operator being under the seats of 
the amphitheater. 

618. Apparatus on one side of the room. Occasionally the 
apparatus is put on one side of the room and instead of projecting 
directly in front of the audience the projection is on one side of the 
room. The auditors simply turn in their seats to face the screen. 




L T 




FIG. 245. SECTIONAL VIEW OF A LECTURE ROOM SHOWING THE POSITION OF 
THE PROJECTION APPARATUS WITH A TRANSLUCENT SCREEN. 

Ml Magic lantern or other projection apparatus on its table (T) and 
raised platform (PI) in a room outside the lecture room. 

L T Lecturer's table. 

Tr Sc Translucent screen. The audience does not see the apparatus; 
only the screen image is visible. 

Lantern slides must be inserted in the holder facing the objective, not the 
light, or the image will have the rights and lefts changed like fig. 213. 

This is not so satisfactory as when the screen is directly in front 
(fig. 243). 

619. Apparatus wholly without the room. Regardless of the 
form of the room, the apparatus may be placed in a room just back 
of the lecture table in front of the audience and a translucent screen 
employed. This arrangement has decided advantages, but a 
translucent screen is not so satisfactory as a white opaque screen 
(see fig. 245). 

620. Special operating room. With the ordinary magic 
lantern and projection microscope the apparatus and operator are 



454 WHITE IMAGE SCREENS [Cn. XII 

usually in the general exhibition room, and there is no special 
boxing or enclosure of the apparatus. But in moving picture 
theaters, where there is some danger from the inflammability of the 
picture films, both the fire underwriters and the municipal regula- 
tions usually require some form of fire-proof operating room. 

IMAGE SCREEN 

621. Next in importance to a suitable room for exhibitions 
with projection apparatus is a good screen upon which to project 
the image. 

No one has ever more briefly and clearly stated the qualities of a 
good image screen than Goring & Pritchard: "It should reflect 
the greatest -possible quantity of light and absorb the least." 
"Every care should be taken to render the surface as smooth, white 
and opaque as it can be made" . . . "inasmuch as the bril- 
liancy and perfectness of the picture will greatly depend on the 
whiteness, and the sharpness of its outline upon the smoothness of 
the screen." The screen should be dull white, never shiny. 

622. Screens of plaster paris upon the wall. A screen ful- 
filling all the requirements just given is a wall coated with a smooth 
finish of pure, fine plaster of Paris. 

623. Painted wall screen. While a plaster of Paris wall 
screen is perhaps the best, a smoothly plastered wall, if properly 
painted, gives almost as good results and is much cheaper. The 
wall, as stated, should be finished as smoothly as possible by the 
plasterers, then it is coated with pure linseed oil if porous, or with 
a mixture of equal parts of linseed oil and turpentine if the wall is 
hard and non-porous. When this is dry, the wall is painted with 
either white lead ground in oil and thinned with turpentine, or 
with "sanitary paint" thinned with turpentine. The sanitary 
paint has the advantage that it does not turn yellow with age, and 
that it is more easily cleaned with soap and water. 

When the paint is properly thinned it should be strained through 
one or two layers of gauze (cheese cloth) to get out any lumps or 
coarse particles. 



CH. XII] WHITE IMAGE SCREENS 455 

In spreading the paint on the wall one should use a soft brush 
and apply only the tip of the brush. This will give a smooth finish 
and if one uses plenty of paint there will be no joints, but the 
whole will appear like one uniform coat. Practical painters call 
this "flowing on the paint." 

After one coat is well dried another can be put on until the wall is 
perfectly white. If plenty of turpentine is used the surface will be 
dull. It should not be glossy or shiny. 

Whenever the surface becomes dirty it can be washed off with 
soap and water. If it is not up to standard whiteness after the 
washing and drying, put on another coat of the paint. 

Sometimes hot glue, 15% to 20% in water, is used for sizing the 
wall. This answers well if the wall is perfectly dry and not subject 
to moisture. In general it is safer to use the linseed oil sizing. 

In our experiments several white paints were used, but the pure 
white lead (sometimes called "flake white") and the non-lead con- 
taining paint called "sanitary paint" were found most satisfactory. 
The latter has the advantage over white lead that it does not yellow 
with age, and gives a very opaque and white surface which stands 
washing with soap and water very well. 

624. Whitewashed wall screens. A smoothly plastered wall 
that has been carefully whitewashed with milk of lime gives a good, 
dull white surface for a projection screen. It rubs off rather easily 
and cannot be cleaned. Of course a fresh coat of whitewash will 
renew the screen. It. is cheap as well as good. One should take 
pains to strain the whitewash, and to apply it smoothly so that a 
uniform surface will be produced. 

We did not find a kalsomined wall satisfactory for projection. 
It is, or soon becomes, too yellow. 

625. Painted cloth screens. A good screen can be made by 
stretching some smoothly woven, strong cotton cloth (strong 
muslin) upon a frame and painting it as for the wall ( 623). The 
frame must be strong and the cloth stretched tight so that there 
will be no wrinkles, and it must not rest against anything. 

One could paint directly on the cloth, but it is more satisfactory 
to size the cloth in some way first. One of the best methods is 



456 WHITE IMAGE SCREENS [CH. XII 

to use white linseed oil, raw or boiled. The oil is put on with a 
soft brush like paint. It is well to make all the brush strokes in 
one direction, so that the lint or nap on the surface of the cloth will 
be smoothed down in one direction. After the linseed oil is dry the 
cloth is painted, preferably with sanitary paint and turpentine, 
although white lead thinned with turpentine answers well. One 
coat should be allowed to dry before adding another. It takes 
from one to two days for each coat to dry. The screen will be 
white and opaque with three to five coats. Care should be taken 
to strain the paint as for the walls ( 623), then there will be no 
rough spots ( 62 $a, 62 sb). 

If the curtain gets grimy it can be wiped off with soap and 
water, and if necessary after it is dry, a fresh coat of the paint can 
be put on. 

626. Roller screens. Cloth screens which have been painted 
as just described make excellent roller curtains, for the sizing and 

62 Sa. Amounts of sizing oil and paint for a cloth screen. For oil-sizing 
and painting a muslin screen the following times for drying in the summer, and 
the following amounts of oil and paint were used to make a perfect screen. 
For sizing, white raw linseed oil was used, and only one coat was applied. 

For this it required 220 cubic centimeters of the linseed oil per square meter 
of cloth, or about one-tenth of this amount per square foot. 

For painting, a preparation of sanitary paint known as "Artists' Scenic 
White," ready for use on screens was used, two coats were applied. It 
required no cc. of the paint for each square meter of surface. 

It required about 36 hours for the raw oil sizing to dry; 24 hours was 
sufficient time for a coat of the white paint to dry. The finished screen was 
flexible and easily rolled. 

For a screen 3 meters or 10 feet square it would require for sizing and paint- 
ing about two quarts of linseed oil and about the same amount of the "Artists' 
Scenic White" or any other white paint for two coats of the paint. 

625b. The cloth may be sized by the use of white shellac. This is thinned 
about half with denatured alcohol and painted on the surface just as described 
for the oil size. It gives a good surface to paint on, but does not leave the 
curtain so flexible. 

A hot 15% to 20% solution of white glue in water may also be used as 
described for the oil or shellac size. This has the advantage of pasting down 
the nap of the cloth and of giving a very good surface to paint on. It has the 
disadvantage of expanding and contracting greatly with different conditions of 
moisture. If the glue size is used the curtain should have at least one coat of 
paint on the back, so that the glue size cannot be so easily affected by moisture. 

625c. The authors wish to express their appreciation for information on 
paints and the painting of wall and cloth screens for projection, to Mr. A. E. 
Nash, Superintendent of the Cornell University paint shop. 



CH. XII] WHITE IMAGE SCREENS 457 

painting leave the cloth flexible, and without liability of cracking 
and peeling. They are mounted on heavy spring rollers as 
ordinary window curtains are so commonly mounted, and can be 
rolled up when not in use. 

627. White cloth screens without paint or other facing. 

White cloth such as a bed sheet has always been and still is used. 
The cloth should be as white as possible, and of good thickness. 
It is also advantageous to have the screen of one piece without 
seams. Bed sheets may be obtained in large dry goods houses 
about 3 meters square (10 ft. sq.) without seams. These make 
very good curtains when the folds are ironed out, and the sheet 
stretched to hold it flat. It is not easy to stretch a sheet so evenly 
that there will be no folds or wrinkles. Fortunately, a slight 
unevenness is not noticeable in the screen image. A screen which 
appears quite uneven to the naked eye in daylight may give very 
good screen images and appear perfectly smooth, when giving an 
exhibition. 

Cloth screens have the disadvantage that they are not suffi- 
ciently opaque. If one goes behind the screen the image is almost 
as well seen as in looking at the face of the screen. This means 
that almost as much light traverses the screen as is reflected from 
the face. Naturally, it takes much more light for a brilliant screen 
image than with an opaque screen ( 632). 

For some purposes it is advantageous to be able to see the image 
on the back, then assistants behind the screen can make the 
appropriate noises to make the scene seem more real. For exam- 
ple, in a moving picture scene, sounds can be made to imitate 
the breaking of the waves on the shore, the clatter of horses hoofs 
on a pavement, etc., etc. Unless the assistant could see the 
image it would not be possible to suit the sound so accurately to 
the scene. 

Sometimes so large a screen is needed that strips of white cloth 
are sewed together. If this must be done the seams should be very 
smooth. On such screens the seams show like lighter streaks on 
the image, as more light is reflected from the double thickness of 



458 WHITE IMAGE SCREENS [CH. XII 

cloth. Behind the screen the seams show as black or dark streaks, 
as less light traverses the screen along the seam ( 627 a). 

628 Paper screens. The suitability of white paper screens 
has been recognized for a long time. One of the best possible 
screens is a large sheet of white cardboard. As shown by photo- 
metric measurements, the reflections from a white cardboard are 
almost as great as from the standard surface of oxide of mag- 
nesium ( 632). The white cardboard is especially suitable for 
the images of the high power projection microscope, and if it could 
be had in sufficiently large sheets it would make an almost perfect 
screen for large rooms. (In large paper stores one can get sheets 
71x112 cm. (28x44 in.). The paper used for drawings by 
architects and engineers and 69 x 102 cm. (27 x 40 in.) in size is 
also excellent for screen purposes. It is not so easy to get a smooth 
surface as with the cardboard). 

Finally, cloth is sometimes faced with paper to give a more 
opaque and perfect screen. 

SCREENS WITH METALLIC SURFACES 

629 Dull white surfaces reflect almost equally throughout 
the whole hemisphere (fig. 248) and therefore the image appears 
almost equally brilliant in any position. Those near the axis of 
the projection apparatus in the middle of the room do not see the 



627a. Screens for traveling exhibitions. When exhibitions must be 
given in school-houses and in halls where there is no lantern and no screen, the 
exhibitor must supply both. In traveling it is inconvenient to carry a roller 
screen, and usually the screen is folded so that it can be packed in a small 
space. This, of course, makes creases in the screen, and besides there is noth- 
ing to support it so that it will hang smooth and even. 

For a traveling screen a heavy, seamless bed sheet is excellent. Bed sheets 
in one piece as large as needed are to be had. To hang these sheets there 
should be a strong cord along the upper edge either in a hem or in curtain rings. 
From the corners of the sheet should be strong cords by which the sheet can be 
stretched out smooth and held in position by passing the cords through screw 
eyes or attaching them to other fixed supports. 

It is well also to have rings along all the edges to attach strings to, to pull 
the edges taut, and to support the curtain at the upper edge. 

For temporary use, a sheet may be stretched and held in position by tying 
strings to the corners and by fastening the strings along the edges by safety 
pins. 



CH. XII] 



SCREENS WITH METALLIC FACIN G 



459 



image much more brilliantly illuminated than those at the side. 
Sir David Brewster in 1832 advocated and used the bright metallic 
surface on the back of looking glasses, which at that time was 
composed of mercury and tin. Later, surfaces covered with 
silver-leaf, silver particles or particles of aluminum have been tried. 
Last of all, plate glass has been ground on one side, and the smooth 
side silvered. The ground surface of the glass is turned toward the 
projection apparatus and facing the spectators who get the image 
reflected from the mat surface of the glass and transmitted from 
the mirror through the mat ( 6 2 pa). 





FIG. 246. DISTRIBUTION OF LIGHT 
REFLECTED FROM A WHITE 

SCREEN. 

It is approximately uniform through- 
out the entire hemisphere. 



FIG. 247. DISTRIBUTION OF LIGHT 
FROM A SEMI-DIFFUSELY RE- 
FLECTING SCREEN. 

The closeness of the arrows indicates 

the apparent brightness as seen 

from different directions. 



630. Suitability of metallic screens. Metallic screens are 
not suitable for micro-projection, or, indeed, for any projection if 
fine details are to be studied close to the screen, but details which 
can be seen at a distance of 2 to 3 meters are very well brought 
out on the mirror screen, and other metallic screens. In com- 
paring a mirror screen, an aluminum bronze screen and one of 
plaster of Paris or cardboard if the image was observed within 
the narrow angle of 15 degrees to the right or left of the axis, 30 
in all, the mirror screen was brightest, the aluminum next, and 
finally the plaster of Paris or cardboard, the screens being in the 
field at the same time so that the comparison was under identical 

629a. The authors wish to acknowledge their indebtedness to The Motion 
Picture Screen Company of Shelbyville, Indiana, U. S. A., for their courtesy in 
sending a sample of their "Mirror Screen" for experiment; to the Bausch & 
Lomb Optical Company for the loan of the two metallic screens of Zeiss; to 
the J. H. Gentner Company of Newburgh, N. Y., for samples of Mirroroide; 
and to other screen manufacturers for courteous answers to inquiries. 



460 



BRIGHTNESS OF SCREENS 



[CH. XII 




CH. XII] TRANSLUCENT SCREENS 461 

On the curved surface of the diagram are given the degrees of inclination of 
the light. On the diameter, and on the radius at right angles to the diameter 
are given the percentage of apparent brightness. Magnesium oxide is taken 
as the standard and called 100%. 

The data shown on the diagram are given in figures in the table, 632. 

Curve i. Screen coated with magnesium oxide. It is to be noted that, it is 
only in the central region that the full 100% of reflection occurs. 

Points 2222 Plaster of Paris screen. 

Curve 3 Cardboard screen. 

Points 4 4 Screen painted with white lead. 

Points 5 5 Screen painted with Artists' Scenic White. 

Points 6 6 Screen painted with zinc white. 

Curve 7 Cardboard screen painted with aluminum. 

Curve 8 Zeiss metallic screen. 

For 9 see the table, 632. 

For the Mirror Screen, see the table, 632. 

Points 10 10 10 Reflection and transmission of a white muslin screen. 
Note its uniformity ( 632). 

Points ii ii ii Reflection and transmission of white gauze (Griswoldville 
gauze, No. 10). With this screen more light is transmitted than reflected. 

Point 12 Transmission of ground-glass. 

Point 13 Reflection of bristolboard. 

conditions. At an angle of 30 degrees and upward the metallic 
screens appeared almost black, and the white screens pure white. 

631. Translucent screens. For the old phantasmagoria and 
for many appearances given by shadow pictures it is necessary to 
have a translucent screen like ground-glass or translucent cloth 
or paper. The paper or cloth is rendered as translucent as desired 
by the use of water, water and glycerine, or oil. Tracing cloth 
makes good translucent screens of moderate size. 

With a translucent screen the apparatus is entirely out of sight 
behind the screen and only the picture shining through the screen 
is seen by the audience. This is not so good and effective a method 
of showing projection images as the opaque white screen or the 
metallic screen, for much more light is lost (fig. 248). It is still 
used in some institutions, as it entirely eliminates the projection 
apparatus and the operator from the auditorium ( 631 a). 

The ground-glass screen is excellent, but this, like a metallic 
screen restricts the brilliant image to a rather narrow angle (see 
630, 632 and fig. 250). The ground surface should be fine or there 
is given the appearance of looking toward a bright light in a snow 
storm, this is especially marked if one is near the ground-glass and 
looking nearly along the axis. 



462 REFLECTION OF DIFFERENT SCREENS [CH. XII 

The cloth screens were not so satisfactory as the ground-glass 
because the crossing threads make a kind of grating and one sees 
diffraction images ; and if one is in direct line with the arc lamp 
the cloth acts almost as if it were transparent. The translucent 
mercerized paper used in making tracings is practically as good as 
ground-glass, but it is difficult to hold it smooth and even. The 
tracing cloth used by architects and engineers is good for a trans- 
lucent screen. 

There is a practical difficulty with all translucent screens. On 
account of the poor reflection, the operator cannot tell with the 
same certainty when the image is in focus as with a white, opaque 
screen. 

632. Table of the reflection of different screens compared 
with magnesium oxide. 

No. At 15 At 45 At 60 

1 Magnesium Oxide 100 83 

2 Plaster of Paris 95.6 88.7 78 

3 Cardboard 84.5 67 

4 White Lead 88.5 79.4 

5 Century Company's White 89.4 81 

6 Zinc Paint 84.4 76.5 

7 Aluminum Paint on Card 210 18 

8 Zeiss Metallic Screen, smooth .... 136 14 

9 Mirror Screen 200 

10 White Muslin, Reflection 73.4 69.1 66.9 

Transmission 39 30 

11 Gauze, Reflection 33 27 

Transmission 35 

12 Ground-Glass, Transmission 300 14.2 

13 Bristolboard, Reflection 91.5 



631a. For example, in the anatomical institute at Munich. Here all the 
projection, whether with the magic lantern, the projection microscope or the 
opaque lantern, is upon a translucent screen; also in some of the lecture rooms 
in Holland. 



CH. XII] 



BRIGHTNESS OF SCREENS 



463 




FIG. 249. DISTRIBUTION OF APPARENT BRIGHTNESS WITH DIFFERENT 
SCREENS WHEN VIEWED FROM DIFFERENT DIRECTIONS. 

On the sides and curved surface are given the degrees of inclination of the 
reflected light. 

The numbers along the central radius indicate the relative brightness of each 
screen, magnesium oxide being used as the standard and called 100%. 

A Magnesium oxide screen. It gives the standard brightness of 1 00% and 
reflects nearly equally throughout the entire 180 degrees. 

B White cardboard. 

C Screen with aluminum bronze facing. This gives 3.2 times the bright- 
ness of magnesium oxide in the center, but it falls off rapidly at the sides. 

E Mirror screen. This gives 7. 1 times the brightness of magnesium oxide 
in the center. 

It is to be noted in general that the mirror screens, (C. E.) give great inten- 
sity when seen near the center, and that this intense light is restricted to an 
angle of about 25 degrees. Farther to the side the light falls off rapidly, 
being in marked contrast with the white screens (A B). 



464 SIZE OF SCREENS CH. XII] 

SIZE OF SCREENS AND SCREEN IMAGES 

633. The size of screen images which will give the best 
results in a given case can only be determined by trial. The size 
should be great enough so that the people sitting on the back seats 
can see all the details to be shown and still not so large that those 
sitting near the front will be repelled by the coarseness of the image. 

As a result of experiments to determine the best size of screen 
picture for the average seat in a room the following general rules 
have been worked out so : 

634. Size of the screen for lantern slides. The screen image 
must be large enough so that details are visible to the most distant 
spectator. For example, in teaching work and in demonstrations 
at scientific meetings, etc., lantern slides often contain tables of 
figures and printed sentences. Naturally, the farthest sitter 
should be able to see the figures and to read the words easily. 

This could not be done by those on the back seats if the letters 
were much smaller than six point. Of course, if the letters on the 
slide are as large as eight or ten point type (fig. 216), they can be 
read at a glance. 

In long, narrow rooms the magnification necessary to enable the 
people on the back seats to see the details well will make every- 
thing gigantic for those sitting near the screen. 

For a well atranged auditorium, if the letters and numerals on the slide are of the size of 
6 point type, such as shown in this sentence, and the screen image is from one-fourth to one-, 
fifth as wide a 1 -, the distance from the farthest seat in the room to the screen, all in the 
audience should be able to read the print on the lantern slide with ease. 

635 Projection objectives necessary to give the proper 
screen image with the magic lantern. If the lantern can be at the 
extreme rear of the room, and the image of the slide is to be one- 
fourth or one-fifth as wide as the room is long, as stated above 
( 634), a projection objective of 30 cm. (12 in.) focus will give the 
desired screen image for a properly made lantern slide, no matter 
what the size of the room. This is because the 30 cm. objective 
gives an image on the screen, regardless of its distance, which will 
appear to the observer standing by the lantern, like the same lan- 
tern slide held 30 cm. (12 in.) in front of the observer's eyes. If 
the lantern slide is well made and properly proportioned all the 



CH. XII] 



BRIGHTNESS OF SCREENS 



465 



details should be plainly visible when the slide is 30 cm. in front of 
the eyes, and therefore are plainly visible in the screen image as 
far back as the lantern. 




FIG. 250. DISTRIBUTION OF BRIGHTNESS OF TRANSLUCENT AND REFLECT- 
ING SCREENS WHEN SEEN FROM DIFFERENT DIRECTIONS 

Reflected Light from Magnesium Oxide and Mirror Screen. 
Light Transmitted through Ground Glass. 

Note that the mirror screen when seen perpendicularly reflects 7 times 
as much light as does magnesium oxide, and ground-glass 19 times as much. 

But this great brightness of the mirror screen and the ground-glass is 
limited to a very narrow angle, while the white magnesium oxide reflects 
nearly equally throughout the entire hemisphere. 

Brightness of MgO is taken as unity and the figures on the radius indicate 
the number of times brighter the screen appears than this. 



4 66 



SIZE OF SCREENS 



[CH. XII 



If the letters and numerals and other details on the slide are too 
small to be seen by the normal eye when held 30 cm. (12 in.) away, 
then they will not show clearly in the screen image with this objec- 
tive at the back of the room, although they may be plainly visible 
to those near the screen. 

As the lantern is frequently not quite at the extreme back of the 
room, an objective of 25 cm. (10 in.) focus is more commonly used 
than the one of 30 cm. (12 in.). It makes the image somewhat 
larger, and for many people is more satisfactory. 

636. Objective to use when the lantern is not at the back of 
the room. Regardless of the position of the lantern a screen image 
must be large enough for all in the room to see the details as stated 
above (633-634). 




FIG. 251. 



SIZES OF SCREENS NECESSARY FOR DIFFERENT 
SCREEN DISTANCES. 



This shows that the same object and objective will give a screen image of a 
size directly proportional to the screen distance. 



CH. XII] SIZE OF SCREENS 467 

If the lantern cannot be at the back of the room, but must be 
closer to the screen, then the projection objective must be of shorter 
focus than 25 to 30 cm. (10-12 in.). 

To determine the proper objective to use to give the desired size 
of image in any case one must proceed as follows : 

1 i ) The size of screen image is decided on by remembering that 
it should be between one-fourth and one-fifth the distance to the 
farthest seat in the room. 

(2) The distance from the screen to the lantern must be 
measured. 

(3) Following the simple optical law founded on the geometry 
of similar triangles that: "The size of object and image vary 
directly as their distance from the center of the objective," one 
can by simple proportion get the focus which the objective should 
have for a given screen image. 

637. Examples. For example, suppose the distance from the 
screen to the farthest seat is 20 meters (66 ft.), the width of the 
screen should be not less than one-fourth this distance, i. e., five 
meters (16.5 ft.). 

Now suppose that instead of the lantern being 20 meters from 
the screen it is only n meters (36 ft.) from it, what should be the 
focus of the projection objective to give a screen image 5 meters 
(16.5 ft.) wide? 

The formula best adapted for this calculation is: 

f d 



where f is the distance of the object from the center of the objective 
(focus of the objective). 

is the size of the object. 

d is the distance from the objective to the screen. 

1 is the size of the screen image. 

It is assumed in all the calculations for the magic lantern that 
the width of the lantern-slide opening or picture is 7.5 cm. or 3 
inches. 



468 SIZE OF SCREENS [Cn. XII 

In the above example 
f is unknown. 

the size of the object is 7.5 cm. 3 in. 
d the distance of the screen image is n meters 36ft. 

1 the size of the screen image is 5 meters 16.5 ft. 
Substituting the values in the formula we have, for metric values, 

f ll 7-S XI1 r r .LI. t.- .,_ 

= or f = - - = 16.5 cm., focus or the objective. 
7-5 5 5 

For English values 

- = or f = - = 6.5 in., focus of the objective. 
3 16.5 16.5 

638. Size of screen image for moving pictures. As the 
scenes depicted by the moving picture are so largely of human 
action, and thus resemble a theater play, one would think that the 
standard should be the representation of people in their natural 
size. The fact is, however, that in most picture theaters the people 
represented are of heroic or semi-heroic size, being from 1^2 to 
two times the natural size of ordinary people. 

The large size of the moving picture on the screen has come about 
naturally, as the details of movement and the facial expression of 

636a-637a. In the formula here given it is assumed that the objective will 
always be at its principal focal distance from the object regardless of the screen 
distance. This is not strictly true, but as the screen distance is so great rela- 
tively to the distance of the objective from the object, the slight error involved 
in the above assumption is negligible. If the screen distance and the principal 
focal distance were more nearly the same, the error would be altogether too 
great to be neglected (see fig. 210). 

It follows, naturally also, from this formula that, if any three of the elements 
are given, the fourth can be found. Ordinarily, it is the proper focus of the 
objective to use that is unknown, but any one of the elements might be desired, 
and it can be found if one knows three of them. 

As it is the focal length of the objective that is most often required, the 
following may be of assistance; it simply states in words what the formula 
shows: 

To find the focal length of the objective needed, the screen distance and the 
size of the screen image being known: Multiply the screen distance in meters 
by 7.5, and divide the product by the size of the screen image in meters and the 
result will give the focus of the objective in centimeters. For English measure : 
Multiply the screen distance in feet by 3 and divide the product by the size of 
the screen image in feet, and the result will give the focus of the objective in 
inches. 



CH. XII] SIZE OF SCREENS 469 

the actors could not be seen if they were only of the size of average 
human beings. On the theater stage the action is made more 
intelligible by the spoken words; but where there is only pan- 
tomine one must see the details of the action and the facial expres- 
sion to make the play fully intelligible. 

To enable those seated in the extreme rear seats to see the action 
on the screen without getting the picture too large for those on the 
front seats, the width of the picture should be between /^ to Ya of 
the distance of the farthest seat to the screen. The width of 
^6 is on the whole the most satisfactory if the end of the room is 
large enough to permit a screen of this size. 

639. The size of the screen limited by the room. It some- 
times happens that the size of the screen which can be used is 
limited by the size of the wall on which it can be placed. The size 
of the screen may also be limited by the height of the ceiling above, 
and the heads of the spectators below. This is true of some lecture 
halls, and of many of the moving picture theaters which are re- 
modeled store buildings. If the screen image is limited in size by 
any of these factors thus requiring a smaller picture than that hav- 
ing a width of ;^th the distance from the screen to the farthest seat 
for the magic lantern or >6th the distance for a moving picture, 
it is necessary to use an objective of longer focus accordingly. 

If the width is limited and one can use any height desired, the 
calculation is made exactly as in the previous section. 

If the height is limited, then the calculation is made in the same 
way except that the height of the object instead of its width is 
taken; that is, for lantern slides the extreme opening of the mat is 
taken as 7 cm. (2^4 in.), or for moving pictures 23.08 mm. long, 
17.3 mm. high, 2 %2 in. X 87 /i2s in. (see 57oa). 

For example, in a university lecture room the greatest height of 
the screen which could be used was 2.9 meters (9.5 ft.), and the 
room was 14.3 meters (47 ft.) long. The question was: What 
focus of objective would give this size of screen image with the 
lantern at the back of the room? 



47 SIZE OF SCREENS [Cn. XII 

In this example 

f is unknown, (i. e., the focus of the objective). 

the height of the object is 7 cm. (2^4 in.). 

d the distance of the screen is 14.3 meters (47 ft.). 

1 the size of the image is 2.9 meters (9.5 ft.). 
Substituting the values in the formula we have : 
For metric values : 

or f = -^- = 34.5 cm., focus of the objective. 
7 2.9 2.9 

For English values : 

or f = -^ = 1 3. 6 in., focus of the objective. 



2-75 9-5 9-5 

An objective of 13.6 in. or 34.5 cm. would have to be specially 
constructed. Those on the market and easily procurable were of 
30 cm. (12 in.) and 38 cm. (15 in.). The shorter focus objective 
gave considerably too large a screen image and could not be used, 
therefore the one of longer focus was taken, and a correspondingly 
long focus condensing lens used. 

Second example. In another lecture room the lantern must be 
16.75 meters from the screen and the screen could not exceed 3.35 
meters in width, what should be the focus of the objective and the 
second element of the condenser to meet these conditions? 

Applying the formula : 

- = '-^- whence f = 37.5 cm. the focus of the objective needed. 
7-5 3-35 

In English measure : 

- = - whence f = 1 5 in. That is, a 1 5 inch objective is demanded. 
3 ii 

640. Size of screen and screen images for micro-projection. 

Here the law holds, that to be satisfactory, the details to be 
shown must be large enough so that they can be seen with ease. 
The microscopic specimens vary so greatly in character that no 



CH. XII] TROUBLES WITH ROOMS AND SCREENS 471 

general rule can be given for the size of screen necessary. For 
large halls the screen used for the magic lantern usually answers. 
In small rooms for special demonstrations it is advantageous to 
have a movable screen on a stand that can be varied in distance for 
different conditions. The magnification and the objective neces- 
sary for the same must be determined in each case by the lecturer 
before the lecture or demonstration (see 400, Ch. IX.) 

641 . Troubles with Rooms and Screens : 

1. Poor image on the screen. This may be due to 

(a) Insufficient light from the radiant; 

(b) Too much light in the room; 

(c) A poor screen dirty or thin; 

(d) If an approximately square room is used, then the 
mirror and other metallic screens will appear very dark and un- 
satisfactory for the spectators outside of an angle of greater than 
15 to 20 degrees from the axis, and the farther outside the 15 de- 
gree position the darker will appear the screen image (fig. 247). 

(e) The objective and second element of the condenser may be 
improperly proportioned, i. e., focal lengths too different ( 89-90). 

2. Oppressive in the room. Too little fresh air. 

3. Room lights shining in the eyes of the spectators. Not 
properly placed or shaded. 

4. Distorted image. The screen and the axial ray from the 
projection apparatus not at right angles. 

5 . The details of the picture not visible for the spectators on the 
back seats. The objective is of too long a focus and it does not 
magnify enough. Use a shorter focus objective. 

6. The screen picture altogether too large. Too short a focus 
objective; use one of longer focus, and adapt the condenser to it. 

7. There is a glare in the room from the ceiling or walls or both. 
The paint used in finishing is shiny, not dull and flat. Use more 
turpentine and less oil in the paint. 

8. The room too dark. Use more room lights properly placed 
and shaded. 



472 



DO AND DO NOT WITH ROOMS AND SCREENS [Cn. XII 



642. Summary of Chapter XII: 
Do 



Do NOT 



1. Use a room properly 
equipped for projection if good 
results are expected ( 602). 

2. Use light-absorbing tints 
for tinting and decorating the 
projection room ( 604) . 



3. Make all paints dull or 
flat, never shiny, for a projec- 
tion room. 

4. Light the projection room 
sufficiently, so that the specta- 
tors can find their seats without 
trouble ( 605). 

5. A perfectly darkened room 
is only necessary for special 
projection ( 608). 

6. Lamps for general lighting 
should be shaded or so arranged 
that their light cannot shine 
directly in the eyes of the spec- 
tators or upon the image screen 
(606). 

7. Have red lights near all 
exits ( 607). 

8. Take the necessary pre- 
cautions to prevent light enter- 
ing the room at the edges of the 
window shades ( 611). 



1. Do not expect good pro- 
jection in a room not equipped 
for it. 

2. Do not use light-reflecting 
colors like yellow, white, light 
red or green for decorating 
the projection room; but the 
dark, rich, light-absorbing col- 
ors, dark red, brown, etc. 

3 . Do not use paint that gives 
a shiny or enamel surface, for 
this will produce a glare by the 
reflections. Use flat paint. 

4. Do not have the projection 
room darker than necessary. 



5. Do not attempt the most 
difficult projection unless the 
room can be made perfectly 
black. 

6. Do not use unshaded 
lamps for the general lighting. 
Light shining directly in the 
eyes of the spectators is very 
distressing. 

7. Do not fail to have red 
lights by the exits. 

8. Do not leave the window 
shades without protection at the 
margins. 



CH. XII] DO AND DO NOT WITH ROOMS AND SCREENS 



473 



9. If possible place the pro- 
jection apparatus at the back of 
the room (612). 

10. The axial ray of the image 
beam should strike the screen at 
right angles (614). 

1 1 . Incline the screen if neces- 
sary (614). 

12. Light the black-board by 
lights behind a curved, metal 
shield (fig. 240). 

13. If a translucent screen is 
used the objects must be put 
into the apparatus with the pic- 
ture facing the objective (516). 

14. Use a good screen ( 621). 

15. If you use a mirror or 
metalliic screen remember that 
it does not reflect equally 
throughout the 180 degrees 
( 629). 

1 6. Make the screen image 
large enough so that the most 
distant spectator can see the 
details (633, 639). 



9. Do not place the projec- 
tion apparatus in the middle of 
the room if it can be avoided. 

10. Do not let the axial ray 
strike the screen obliquely. 

1 1 . Do not incline the screen 
unless necessary. 

12. Do not try to use the 
blackboard without lighting it 
by hidden lights. 

13. Do not forget the rules 
for erect images when using a 
translucent screen. 

14. Do not use a dirty screen. 
Wash it or give it a fresh coat of 
white paint. 

15. Do not use a mirror or 
metal faced screen in a square 
room ; such screens are not good 
for the fine details necessary in 
micro-projection ( 360). 

1 6. Do not have the screen 
image too small nor too large. 



CHAPTER XIII 

ELECTRIC CURRENTS AND THEIR MEASUREMENT; 
ARC LAMPS, THEIR WIRING AND CONTROL; 
CANDLE-POWER OF ARC LAMPS FOR 
PROJECTION 

650. Apparatus and Material for Chapter XIII: 
Direct and alternating current ; Voltmeter, ammeter and watt- 
meter for direct and alternating currents ; Shunt generator, motor- 
generator set, mercury arc rectifier ; Polarity indicators ; Arc lamp ; 
Rheostat, inductor, transformer and other ballast; Carbons; 
Water-cell; Insulated wires, flexible cables, insulators, switches, 
separable plugs, caps, etc. ; Fuses and circuit breakers; Wire, iron 
plates, etc., for home-made rheostats. 

651. Historical development of electric lighting. See the 
Appendix. 

ELECTRIC CURRENTS: KINDS AND COMPARISON 

652. Direct current. The earliest electric currents studied 
and experimented with were produced by the voltaic cell. These 
currents have a constant polarity, always flowing in the same 
direction. 

A direct electric current may be produced by a voltaic battery, 
or by a dynamo ( 652a). 

The first installations of electric plants were all for direct current, 
now they are mostly for alternating current (see below). 

The principal use of direct current at present is for trolley cars, 
and other apparatus where variable speed motors are necessary; 
in electrolysis such as charging storage batteries and the decomposi- 
tion of chemical compounds; for electric lighting in some of the 
more densely populated cities and for projection purposes. 

652a. Generator, Dynamo. Generator is a comprehensive term in- 
cluding all means of producing electric currents whether these means be 
chemical or mechanical. 

Dynamo, on the other hand, is a special term denoting a generator by which 
mechanical is transformed into electrical energy; for example, a steam engine 
may give motion to the dynamo and thus produce an electric current. In a 
word, a dynamo is a generator of electricity, but all generators of electricity are 
not dynamos. 

474 




CH. XIII] ELECTRIC CURRENTS 475 

653. Alternating current. This is characterized by flowing 
first in one direction and then in the opposite direction ; the polarity 
is therefore constantly changing. (See 676). 

Alternating current is pro- 
duced only by dynamos. It is 
used especially for the trans- 
mission of power to great dis- 
tances, incandescent lighting, 
arc lighting, for motors and for 

FIG. 252. CONNECTIONS OF A th electric furnace, as in the 
VOLTMETER TO MEASURE THE 

LINE VOLTAGE. manufacture of carborundum 

G Dynamo. and graphite. 

A Arc Lamp. Alternating current has the 

R Rheostat. advantage of being more easily 

Note that the terminals of the volt- pro duced, as the dynamo is Sim- 
meter are connected to the two points * . 
between which it is desired to measure pier; but its great superiority 
the potential difference. In this case lies in the f act that practically 
it is the main supply (across the line). 

without loss it can be stepped 

up or down in voltage by stationary transformers. This makes it 
possible to raise it to a very high voltage (1000 to 100,000 volts) 
for transmission to a distance over wires of moderate size. It is 
then stepped down in voltage before it is used. In this process of 
stepping up or down in voltage the amperage takes the reverse 
direction, so that the product of the volts by the amperes is a 
constant quantity. 

The disadvantages of alternating current for the arc lamp are: 

1 . The arc is not as bright as with the same amperage of direct 
current. 

2. The light is intermittent. 

3. The alternating current arc is noisy. 

ELECTRIC UNITS AND THEIR MEASUREMENT 

654. Electric Units. For the purposes of this book it is 
necessary to refer frequently to electric units, like the volt, the 
ampere, the ohm and the watt; it seems proper therefore to give 
a brief discussion of these units. 



476 DIRECT CURRENT UNITS [Ca. XIII 

DIRECT CURRENT UNITS 

655. The Volt. This is the unit of electromotive force, that 
is the electric force or pressure necessary to produce one ampere of 
current in a circuit with a resistance of one ohm. 

The difference of potential between the two poles of a Weston 
standard cadmium cell is i.o 19 volts. The ordinary battery used 
for ringing door bells has approximately one volt pressure. 

Voltage is a general term representing the pressure in volts in an 
electric circuit. 

If the difference of pressure between the two given points is 
great, then the voltage is said to be high; if the difference is slight, 
then the voltage is low. For example, in projection one might use 
55 volts for the arc lamp, or 220 volts, or 500 volts. Ordinarily 
neither the low voltage of 55 nor the high voltage of 500 or 220 
is used, but an intermediate voltage of no. 

656. The Ampere. This is the unit of current. It is the 
current which will deposit .001118 gram of metallic silver per 
second from a 15% solution of silver nitrate in water. It is the 
current which one volt will maintain in a circuit with one ohm 
resistance (see below). 

Amperage is the term by which is designated the amount of 

current in amperes flowing at 
any given moment. If a large 
amount of current is flowing the 
amperage is said to be high or 
great, if a small amount, then it 
is said to be low or small. For 

example, in projection, the am- 
FIG. 253. CONNECTIONS OF A VOLT- , , . 

METER TO MEASURE THE ARC perage needed for drawing with 
VOLTAGE. th e microscope on the house cir- 

V VcTtnSer. cuit ( 493) is small (3-6 am- 

A Arc lamp. peres) , while for opaque pro- 

R Rheostat. jection ( 289), and for moving 

Note that the terminals of the volt- J . ' . fe 

meter are connected to the two points pictures ( 693) in large halls 

between which it is desired to measure t h e amount of amperage needed 

the potential difference. In this case . . . 

it is the two carbons (across the arc), is great (20 to 100 amperes ). 




CH. XIII] DIRECT CURRENT UNITS 477 

657. The Ohm. This is the unit of resistance to the flow of 
an electric current. It is represented by the resistance, at zero 
centigrade, of a column of mercury 106.3 centimeters long, of uni- 
form cross sectional area, and weighing 14.4521 grams. Such a 
column of mercury will have a cross sectional area of one square 
mm. 

Ohmage is a term analogous with voltage and amperage. It is 
used to designate the amount of resistance in ohms of an electric 
circuit. 

A conductor may have little resistance, as copper, etc., or it may 
have great resistance like German silver. Naturally then copper 
wire is used largely for electric circuits, and German silver wire for 
making resistors or rheostats ( 7240.}. 

658. The Watt. This is the unit of activity and is the rate 
at which work can be done by a current of one ampere under a 
pressure of one volt. One watt means the doing of work at the 
rate of io 7 ergs per second, or one joule per second. This is approx- 
imately equal to the lifting of i kilogram, io centimeters every 
second. 

659. Kilowatt. A kilowatt is 1,000 watts. This term is 
more common than watt. It is equal to 1.34 horse power. 

660. The watts which any direct current represents are 

obtained by multiplying the quantity of current flowing by the 
pressure that is, the amperes by the volts. Thus, if there were 
an amperage of one and a voltage of one, there would be an 
activity of one watt. If the voltage were io and the amperage 100, 
or the voltage 100 and the amperage io, there would be an activity 
of 1,000 watts, or one kilowatt. 

661. Kilowatt-hour. This is the unit of electrical energy or 
work, which is in commercial use and which is used as a basis for 
making the charges to consumers. A kilowatt-hour is the work 
represented by one kilowatt when acting for one hour. 

In order to find the amount of work done by an electric current 
it is necessary not only to know the rate at which the work is being 
done but also the time during which this rate is continued. Thus, 



478 ELECTRIC MEASUREMENTS [Cn. XIII 

take the example of an arc lamp which uses 20 amperes direct 
current from a no volt line. The line then supplies 20 x no = 
2,200 watts or 2.2 kilowatts. If this arc were used for only a few 
minutes, the energy supplied would be comparatively small, but if 
the arc were used all day, the energy supplied and hence the coal 
or other fuel consumed in generating this power would be compara- 
tively large. In order to measure this energy, the power measured 
in kilowatts is multiplied by the time the power is used. In the 
above example, if the arc were run for eight hours the electrical 
energy used would be 2. 2x8 = 17.6 kilowatt-hours. 

ELECTRIC MEASUREMENTS: VOLTMETERS, AMMETERS, WATT- 
METERS FOR DIRECT CURRENT 

662. Voltmeter for direct current. This is an instrument for 
measuring in volts the difference of potential between two points 
of an electric circuit. 

The voltmeter must be adapted to the kind of current direct 
or alternating and for the pressure, low voltage or high voltage. 
It consists of a delicate galvanometer of exactly the same type as 
that for an ammeter, but it has a high resistance in series with it. 
This high resistance allows but a small current to flow through the 
galvanometer; and this small current is proportional to the differ- 
ence of pressure or voltage between the binding posts of the volt- 
meter, and causes the needle of the voltmeter to be deflected. 
Numbers on the dial indicate the voltage for different amounts of 
the deflection. 

663. Connection of the voltmeter with the circuit to be 
measured. One pole of the voltmeter is positive and one negative. 
To connect the instrument with the circuit for determining the 
voltage between two points, the positive binding post of the volt- 
meter is connected by a wire to the positive point in the circuit, and 
the negative binding post with the negative point in the circuit 
(fig. 272). This gives the full electric pressure between the two 
points connected with the voltmeter, although only a very small 
current flows through it on account of its high resistance. The 



CH. XIII] 



ELECTRIC MEASUREMENTS 



479 



main current continues to flow in the electric circuit between the 
two points exactly as though the voltmeter were not in use. 

The voltmeter should not be connected in series with the line as 
with the ammeter ( 665). While no particular harm might be 
done, the high resistance of the voltmeter would allow only a small 
current to flow in the line and if one were using an arc lamp it 
would go out from the insufficient current. 

If one does not know the direction of the flow in the circuit to be 
tested, the voltmeter can be correctly connected by trial as follows : 
Connect the positive binding post of the voltmeter by a wire to one 
of the points, and the negative binding post by a wire to the other 
point. Turn on the current, and if the connection is right the 
needle of the instrument will point to the voltage ; if the connection 
is wrong the needle will tend to be deflected off the scale below the 
zero point. If it is wrong, turn off the current and reverse the 
position of the wires in the binding posts. 

664. Ammeter. This is 
an instrument to show the 
amount of current flowing 
through a given circuit at a 
given instant. It consists of a 
galvanometer of the particular 
type adapted to the current 
used, that is, direct or alter- 
nating current. It is also 
adapted to the amount of cur- 
rent to be measured, that is 
small currents and large cur- 
rents, say from o to 10, o to 25, 
o to 50, o to 100, etc. 

The galvanometer part of the 
ammeter is a delicate instru- 
ment so that the whole current 
used in projection is not sent 
through it, but a definite frac- 
tional part goes through it and 




FIG. 254. CONNECTING AN AMMETER 

IN THE LINE TO MEASURE THE 

CURRENT FLOWING. 

a Ammeter, A, with external shunt, 



S. 

b Ammeter with self-contained 
shunt. The shunt in this type is in- 
side of the instrument case. 

Note, the ammeter is connected 
along one wire so that the entire cur- 
rent flows through the instrument and 
its shunt. 



480 ELECTRIC MEASUREMENTS [Cn. XIII 

the main part of the current goes through a special wire, known as 
a shunt (fig. 254). 

In some ammeters the galvanometer, and the shunt are in the 
same box (self-contained ammeters) , in others the shunt is outside 

(fig- 254). 

When an electric current flows through the ammeter, the 
galvanometer needle is deflected, the amount of the deflection 
measuring the amount of the current. With the ammeters used 
in projection, the galvanometer has been calibrated so that the 
needle points to the number of amperes of current flowing in a given 
case (fig. 145). 

665. Connection of the ammeter with the projection circuit. 

If one is to use an ammeter in an electric circuit, the instrument is 
connected with the line in series, that is along one wire. Further- 
more, it is necessary to connect the positive pole of the ammeter 
with the positive end of the wire, and the negative end with the 
negative pole. In most cases when installing a projection outfit 
the direction of the current flow is not known, and the proper 
connection of the apparatus is found by trial (see 702 for the 
direct current arc lamp). 

To install an ammeter cut one of the wires, and insert one cut end 
in the positive, and the other cut end in the negative binding post 
of the ammeter. Then the arc lamp and the rheostat are wired as 
shown in fig. 270. 

Now close the switch and cause the arc lamp to burn. If the 
ammeter is correctly connected, the needle will point to the number 
of amperes of current flowing. If the connection is wrong, then 
the needle will tend to move off the scale below the zero mark. In 
case the connection is wrong, open the switch and reverse the posi- 
tions of the wires in the binding posts of the ammeter. When the 
current is turned on again the needle will be deflected until it 
points to the number of amperes. 

By looking at fig. 273 it will be readily seen why one of the cut 
ends of the same wire will be positive and why one will be negative. 
That is, if the whole circuit is considered from the dynamo back to 
the dynamo, it will be seen that starting from the positive pole of 



CH. XIII] ELECTRIC MEASUREMENTS 481 

the dynamo, any point in the circuit toward or nearer this positive 
pole will be positive in comparison with any other point nearer the 
negative pole. Then if the circuit is cut at any point the end of 
the wire next the positive pole of the dynamo will be positive, and 
the end nearer the negative pole of the dynamo will be negative. 
Now if the cut end of the wire nearer the positive pole of the dynamo 
is inserted in the negative binding post of the ammeter, and the 
other end in the other binding post, the needle tends to be deflected 
in the wrong direction. If the two ends of the wire are correctly 
connected with the ammeter, the needle will be deflected in the 
right direction, and indicate the amperage. 

666. Ammeter for projection. In projection the ammeter is 
usually all that is required, for the voltage on a given line is nearly 
constant, and can be found easily by inquiring of the central station. 
On the other hand, the required amount of current for different 
purposes varies greatly and the factors in the production of a good 
image are so many that an ammeter to show at a glance what 
amount of current is flowing is of the highest importance, for with 
a given amount of current the operator knows at once what kind 
of a light can be reasonably expected in the different cases. If the 
screen light is not good with the adequate amperage for the purpose 
then he can look to the other possible causes of failure (see 61-96). 

If one is to be able to determine for himself all the electric fac- 
tors in projection work, then a voltmeter and a wattmeter should be 
added to his apparatus. 

667. Precautions for the ammeter. In connecting the am- 
meter be sure not to connect the ammeter directly to both line 
wires. As the ammeter has very little resistance, putting it across 
the line would have practically the same effect as connecting the 
two points with a heavy wire, that is a short circuit would be 
formed and the fuses would be blown. Besides the very heavy 
current which would flow momentarily might be sufficient to 
seriously damage the delicate instrument. 

668. Safe rules for the beginner to follow when connecting 
instruments may be stated as follows: 



482 ELECTRIC MEASUREMENTS [Cn. XIII 

For the voltmeter. After all the connections for the circuit are 
complete, connect the two terminals of the voltmeter to any two 
parts of the line between which it is desired to measure the differ- 
ence of potential. 

For the ammeter. After all the connections for the circuit are 
complete, and after the arc has been found to work satisfactorily, 
cut one of the wires and insert the self-containing ammeter in 
between the two cut ends just as if it were a short piece of heavy 
wire. If there is an outside shunt connect the ends of the supply 
wire to the large binding posts of the shunt and the wires of the 
ammeter to the smaller binding posts of the shunt. 

669. The Wattmeter. This is an instrument for measuring 
electrical power or activity. There are two types of wattmeter 
the portable or indicating wattmeter and the integrating or supply 
wattmeter. Both work on the same principle, but the method of 
indication is different. 

The wattmeter has two sets of terminals or binding posts. One 
set is connected with the line in series along one wire like an 
ammeter, while the other set is connected in multiple that is, to 
both lead wires like a voltmeter. In fact this instrument is a 

sort of a combination voltmeter 
and ammeter, as it measures 
the product of the volts times 
the amperes. 

In connecting the wattmeter 
FIG. 255. WATTMETER TO MEASURE p. rea t care must be taken to ret 
THE POWER CONSUMED AT THE ARC. 

G Dynamo. the sets of binding posts correct- 

W Wattmeter. ly joined with the line. That is 

R Rheostat. the binding posts for the current 

The heavy line wire passes to the terminals of the wattmeter must 

wattmeter and from it to the upper car- be con nected in series or along 

bon. From the lower carbon the heavy 

wire passes to the rheostat and back one Wire like the ammeter (fig. 
to the dynamo. The fine wire passes 2 _ 3 ) while the vo l ta ge binding 
from the upper carbon to the watt- 
meter, and from the wattmeter to the posts must be connected in para- 
lower carbon. With this connection of llel or across t he line, like a 
the wires the power consumed at the 
arc is measured. voltmeter (fig. 272). 





CH. XIII] ELECTRIC MEASUREMENTS 43 

If the wattmeter were wrongly connected, the instrument could 
not register the watts on the one hand and on the other it might be 
injured. 

670. Portable wattmeter. This has a pointer which shows 
directly in watts, the power consumed at a given instant. 

671. Stationary or house 
wattmeter. The wattmeter for 
the electric supply looks some- 
thing like a gas meter for the gas 
supply. It is of the integrat- 

FIG. 256. WATTMETER TO MEASURE in g ^P 6 - is permanently con- 

THE POWER DELIVERED BY THE nected with the line, and con- 

DYNAMO. taing & wheel the ed of w h OS e 

G Dynamo. . , 

W Wattmeter. rotation is directly proportional 

A Arc lamp. to fa e pow er consumed. This 

R Rheostat. 

The fine wire connects the watt- wheel turns pointers over the 
meter with the line where the power is dials on which are indicated 

kilowatt-hours. The numbers 

toward which the pointers are directed indicate the kilowatt- 
hours which have been used, just as the pointers in a gas meter 
indicate the number of cubic feet of gas which have been used. 
For example, by consulting the wattmeter before and after an 
exhibition one can see how much work, measured in kilowatt-hours 
has been consumed by the arc light. 

Suppose the voltage of the line were no, and the voltage between 
the carbons is 55 volts. Suppose the amperage is 20, then the 
watts should be (volts times amperes) 55x20 = noo watts at any 
instant, and for an hour, for example, it would be 1 100 watt hours, 
or i.ioo kilowatt-hours. 



67 la. With both direct and alternating current, when a rheostat is in 
the circuit, the amperage may be found by the aid of the stationary wattmeter, 
this is always present in a house supply of electricity, as is the gas meter for the 
gas supply, and one does not always possess an ammeter. 

It is necessary to know the voltage of the line. This is usually 1 10 or 220. 

One must also know the watts, or kilowatts at any given instant. This can 
be found by the wattmeter as follows: Suppose the reading is 1.87 kilowatt- 
hours. As this number was obtained by multiplying volts x amperes x time, 
and the time is one hour, then the kilowatts of power consumed is 1.87. The 



484 ALTERNATING CURRENT UNITS [Cn. XIII 

UNITS AND THEIR MEASUREMENT WITH ALTERNATING CURRENT 

With alternating current there is, strictly speaking, no such 
thing as voltage and amperage as the electric potential is varying 
from instant to instant. Consequently a kind of average value of 
the electric pressure and amount of current is used instead. 

672. Alternating current voltage. When alternating current 
is measured, the voltage indicated on the voltmeter is the mean 
effective voltage. 

In order that this average effective value for a volt shall corres- 
pond as nearly as possible to the analogous value with direct cur- 
rent, the value taken is the square root of the average of the squares 
of the instantaneous values of the potential difference during an 
entire cycle. Or briefly, it is the root mean squares of the instan- 
taneous pressure. 

673. Alternating current amperage. The number of amperes 
indicated on an ammeter when using alternating current represents 
the mean effective amperage. The average effective value of the 
ampere is, as with the volt, the square root of the average of the 
squares of the instantaneous values of the current during an entire 
cycle. 



voltage with a rheostat is the line voltage. Now as the kilowatts are the pro- 
duct of voltage by amperage divided by i ,000 and both the voltage and the 
kilowatts are known the amperage can be found by multiplying the kilowatts 
by 1,000 to reduce them to watts, and dividing the watts by the voltage = 1 10. 
1870 -H 1 10 = 17 amps. With alternating current, if an inductor (choke-coil) 
is used for regulating the current, the wattmeter can also be utilized for deter- 
mining the amperage at the arc, for by experiment it is known that no matter 
what the line voltage is, the voltage across the arc is usually about 34 volts. 
The fall of potential across the inductor does not count. The wattmeter only 
records the power consumed by the lamp. The amperage, assuming the same 
number of watts as in the above example, would be found this: 1870 -=- 34 = 
55 amperes. That is, with an inductor in place of a rheostat one could use 
several times the amount of current and use only the same number of kilowatts 
of power. As it is the power consumed that must be paid for, one can appre- 
ciate the saving by using an inductor or choke-coil rather than a rheostat. 

The two cases just given are the only ones in which the wattmeter can be 
used to find the amperage. If a current-saver, transformer, rectifier, or other 
similar device is used in the circuit the amperage in the arc cannot be deter- 
mined by the wattmeter, one must use an ammeter of the proper type for the 
current. 



CH. XIII] ALTERNATING CURRENT UNITS 485 

674. Watts with alternating current. With alternating as 
with direct current, the instantaneous watts are equal to the pro- 
duct of the instantaneous volts by the instantaneous amperes. 

As the voltage and amperage with alternating current vary 
from instant to instant over the entire cycle, it follows that the 
instantaneous watts must also vary from instant to instant. To 
obtain the average watts over an entire cycle, the arithmetical 
mean of the instantaneous watts is taken. This average of the 
watts may be anything between zero and the product of the mean 
effective volts times the mean effective amperes, depending on the 
character of the circuit, i. e., whether the circuit contains resistance 
only or whether it contains both resistance and inductance. 

675. Power factor. When alternating current is used with 
inductance in the circuit as described in 736 (where an inductor 
or choke-coil is put in series with the arc) the power transformed 
into heat or work, and hence which must be supplied to the dynamo 
by coal or other fuel is less than the product of the mean effective 
volts by the mean effective amperes. This is because most of the 
energy required to magnetize the iron core of the inductor when the 
current is increasing is returned to the line when the current is 
decreasing. In the case mentioned the line voltage was no; at 
the arc the voltage was 34, and 55 amperes were drawn. The power 
consumption at the arc, which is unable to return any absorbed 
energy to the line, is the product of the volts by the amperes, i. e., 
34 x 55 = 1,870 watts. In this case the power factor is unity. In 
the case of the entire circuit, however, by multiplying the line 
voltage by the amperage, i. e., 1 10 x 55 we get 6050. A wattmeter 
would register only the 1870 watts consumed at the arc. The 
power factor is the value by which we must multiply the product of 
volts x amperes in order to get watts. Thus, if we multiply 6050 
by .3 1 we get 1870. The power factor is of course obtained in prac- 
tice by dividing the watts by the product of volts by amperes, i. e., 
P. F. = Watts -4- Volts x Amperes; and Watts = Volts x Amperes x 
Power factor. Nothing comparable to this effect is possible with 
direct current, that is, with direct current the power factor is always 
unity. 



486 DYNAMO FOR ARC LAMPS [Cn. XIII 

676. Cycle. With alternating current where the current 
flows first in one direction and then in another with a change in 
polarity for each reversal, a cycle includes a change in polarity to 
the opposite, and back to the starting point. That is, a cycle 
includes flow in two directions and consequently includes two 
polarities ; and this is repeated over and over again. 

677. Frequency. The number of cycles per second with an 
alternating current is called its frequency. The frequencies in 
most common use are: 25 cycles, 60 cycles and 135 cycles per 
second. The 60 cycle frequency is most generally used for lighting 
circuits and the 25 cycle frequency is mostly employed for long 
distance transmission, and frequently for motors. The 130 or 135 
cycle frequency is now uncommon. 

SPECIAL DYNAMO FOR ARC LAMPS 

678. The characteristics of the arc are that the potential 
difference between the electrodes is dependent upon the arc length 
but not upon the current (see 743). It is required to supply this 
arc with a constant current regardless of the differences in arc 
length. This may be done with a constant potential supply by 
using a rheostat in series with the arc, or it may be done by using a 
constant current generator. Since the early days of arc lighting, 
street arcs have been connected in series and are supplied by a 
direct current dynamo of this type, no resistance being used. These 
dynamos have an automatic controlling device which increases the 
voltage when the current falls slightly below the rated value (6.6 
amperes) and which decreases the voltage should the current rise 
slightly above this value. For street lighting this regulation must 
be very close, but for projection purposes the regulation need be 
only approximate. There are some types of dynamos which have 
the proper characteristics to be connected directly to an arc lamp 
without intervening resistance. The characteristics of such a 
dynamo must be that a slight momentary increase in current 
caused by a lowering in the potential difference at the arc will be 
met by a decrease in the voltage generated, and conversely a 



CH. XIII] DYNAMO FOR ARC LAMPS 47 

decrease in current will be met by an increase in the voltage 
generated. 

679. Shunt generator. The connections for a shunt genera- 
tor or dynamo are shown diagrammatically in fig. 257. A is the 
revolving armature from which the current is drawn. N and S are 
the poles of the field magnet and F is the field coil which keeps it 
strongly magnetized. The stronger the magnetization of this field 
magnet the higher the voltage furnished by the machine. As 
usually operated the field rheostat R must be continually adjusted 
so that the right current is supplied to the field coil F to keep the 
machine at the desired voltage. 

680. Adaptability of a shunt generator for direct connection 
to an arc lamp. If instead of continually adjusting the rehostat R 
so that the dynamo will supply a constant potential, the machine 
is left to itself it will be found that when no current is supplied, i. e., 
the dynamo is running on no load, the potential difference between 
the terminals a and b is greatest and consequently the current 
flowing in the field coil F is greatest. If now current is drawn from 
the dynamo the potential difference between a and b will drop 
slightly. This will result in a decrease in the current flowing in the 
field coil F, a decrease in the magnetization of the field magnets and 
hence a decrease in the voltage generated. The result is in the 
direction desired, namely, that an increase in the current will be 
met by a decrease in the voltage. 

Whether or not a shunt generator connected directly to an arc will 
work satisfactorily, or whether the arc will be unstable and want to 
either "run away" or "die out" will depend upon the details of the 
design of the dynamo ; that is, the voltage at no load, the resistance 
of the shunt field coils and the resistance of the armature and also 
on the resistance of the wiring to the arc. Some dynamos have 
been designed which will work satisfactorily when connected 
directly to the arc without any intervening resistance. Such 
dynamos may be run directly by some form of engine or they may 
be part of a motor-generator set in which high voltage, direct 
current or alternating current is used to furnish the power. (See 
also 682, 684). 



488 



DYNAMO FOR ARC LAMPS 



[Cn. XIII 




FIG. 257. 



SHUNT GENERATOR CONNECTED DIRECTLY TO THE ARC LAMP 
WITHOUT INTERVENING RESISTANCE. 



N S Poles of field magnet. 

A Armature rotating between the poles of the field magnet. 

a and b Terminals of the Dynamo. 

F Field coil; current through this coil magnetizes the iron of the field 
magnets. 

R Adjustable field rheostat controlling the current flowing through the 
field coil. 

L Arc lamp. 

+ and indicating the polarity of the wires connected to the arc. 

This will maintain a uniform current in the arc regardless of its length in 
case the dynamo is properly designed and proportioned for the purpose. 



CH. XIII] 



CURRENT RECTIFIERS 



489 



CURRENT RECTIFIERS 

681 . While alternating current is mo/e cheaply generated and 
transmitted, especially if the distance is great, the available light 
given by the alternating arc is much inferior to that given by a 
direct current, as can be seen by consulting the table of available 
candle-powers for different amperages ( 756). On this account 
and from the noiseless character of the direct current arc, efforts 
have been made to utilize alternating current to get direct current. 

Up to the present time two methods of doing this for projection 
purposes have proven themselves successful : 




FIG. 258. MOTOR-GENERATOR SET. 
(Cut loaned by the General Electric Co.). 

The alternating current motor is at the left, the direct current generator is 
at the right. The two armatures are mounted on the same shaft. 

682. Motor-generator sets. This is an indirect way of 
getting direct current from alternating. It consists of an alternat- 
ing current motor and a direct current dynamo attached to the 
same shaft. The alternating current is not converted into direct 
current but is used to furnish mechanical power which drives the 
direct current dynamo just as it could be driven by a water-wheel, 
a gas or other engine. 

The efficiency of a motor-generator is about 60%. 

If the dynamo is specially designed for the purpose, the arc lamp 
can be connected directly to it without using a rheostat so that 
there is no loss from this cause as must be the case when the rheo- 
stat is used. (See above 680). 



490 



CURRENT RECTIFIERS 



[Cn. XIII 



683. Mercury arc rectifier. This is a method of securing 
direct current from alternating. It is a utilization of the mercury 
arc, and gives an efficiency of about 70%. The current is slightly 





FIG. 259 FIG. 260 

FIG. 259. MERCURY ARC RECTIFIER, FRONT VIEW. 

(Cut loaned by the General Electric Co.). 

This size is designed for 30 amperes. It requires 14.5 amperes alternating 
current at 220 volts or 29 amperes at 1 10 volts, and delivers 30 amperes direct 
current at 62 volts. (See tests 754). It consumes 2600 watts alternating 
current and delivers 1860 watts direct current which gives 8,600 candle-power 
with the projection arc. 

FIG. 260. MERCURY ARC RECTIFIER, REAR VIEW. 

(Cut loaned by the General Electric Co.). 

This gives a good view of the rectifier bulb and the inductor directly below 
the rectifier bulb which serves to limit the current in the arc by acting upon the 
alternating current primary. The iron case on the floor contains a com- 
pensating reactance which serves to smooth out the fluctuations on the rectified 
current. 



CH. XIII] 



CURRENT RECTIFIERS 



491 




FIG. 261. MERCURY ARC RECTIFIER, DIAGRAM OF CONNECTIONS. 

(Cut loaned by the General Electric Co.). 

The alternating current supply comes in at the upper part of the transformer. 
This supplies alternating current at 220 volts (for a no volt arc) between the 
points C and H. The arrows indicate the direction of flow of the current dur- 
ing one-half of the cycle and the arrows enclosed in circles indicate the flow of 
current during the other half of the cycle. Taking the time when H is the 
positive pole of the transformer, the current flows down this wire and over to 
the point A. Here the current flows through the tube to the cathode B, 
through the battery J (or the arc lamp situated at J) to D. It then flows to 
the right through E and up to G. 

When the current is reversed, current cannot follow this path because 
between A and B the rectifier tube acts as a valve, as the mercury arc allows 
current to flow towards B but never away from it, hence the current must flow 
from G to A l to B through J to D, through the coil F to the left and up to the 
point H. 

The function of the coils E and Fis to act as an auto-transformer, for without 
them current could flow directly from G to H without passing through the 
rectifier tube. In actual practice both coils E and F are wound on the same 
iron core. 



492 CURRENT RECTIFIERS [Cn. XIII 

The small electrode in the bottom of the tube, at C is used in starting the 
tube. In starting, the tube is first rocked making and breaking a mercury 
contact. A small amount of current flows through between C and B and starts 
the arc going, after which it will continue to burn as long as B is the cathode, 
but if the arc is extinguished even for an instant, it will go out and the tube 
must be tilted again before it will work. 

pulsating, but the current is always in one direction and the pulsa- 
tions are so slight that the crater of the positive carbon remains 
almost as constant as with the direct current furnished by a motor- 
generator set. 

Both the motor-generator set and the mercury arc rectifier are 
necessarily expensive. For a small plant to be used much of the 
time for the arc lamp, and where power is needed for other pur- 
poses, like the lighting of the house, pumping water, running 
machinery, etc., etc., it would be cheaper to install one of the 
modern forms of engines. The cost of running these is relatively 
very little, much less than for the current supplied to the rectifier 
or for the motor-generator set. It is also very easy to care for the 
modern engine used with the generator. 

By adapting the generator set for low voltages (60 volts) it is 
possible to connect the arc lamp directly without a rheostat, thus 
saving the energy wasted by heating the rheostat. A rheostat 
may also be used but if so it is called upon to give very slight reduc- 
tion in voltage, and therefore uses up but little energy. 

PROJECTION WITH 135 CYCLE AND 25 CYCLE CURRENT 

684. In most places where alternating current is used for 
lighting, the supply has a frequency of 60 cycles per second, and 
in this chapter it has generally been assumed that the alternating 
current has this frequency. There are, however, places in which 
the supply has a frequency of 135 cycles per second and there are 
others, especially small towns in the neighborhood of large hydro- 
electric plants, in which the supply has a frequency of 25 cycles. 
The authors of this book have had practically no experience with 
other frequencies than 60 cycles. We have reason to believe how- 
ever, that with 135 cycle current the arc will give as good results as 
with 60 cycles and will perhaps have less tendency to show a flicker, 



CH. XIII] 



CURRENT RECTIFIERS 



493 



especially when used with moving picture projection. When 25 
cycle current is used directly (is used raw) to supply the arc, the 
result is very bad. The screen shows a violent flicker. The 
general appearance is much the same as when a pan of mercury is 
jarred rapidly, the surface appears covered with ripples. This 
effect is naturally very trying to the eyes. 




FlG. 262. OSCILLOGRAMS OF THE ALTERNATING CURRENT SUPPLY AND THE 
DIRECT CURRENT DELIVERED. 

(Cut loaned by the General Electric Co.; made from the original photograph'). 

Curve A The direct current delivered. 
B The direct current zero line. 
C The alternating current voltage curve and its corresponding zero 

line. 

The height of the curve A above its zero line B represents the instantaneous 
value of the direct current. Note that while there are slight fluctuations in the 
current, i. e., it is slightly pulsating, the current is always in the same direction 
and that these fluctuations amount to only about 30% of the average value. 
Note also that the maximum current value corresponds to a maximum positive 
value or to a maximum negative value of the alternating current voltage as 
shown in curve C given just below. 



494 



CURRENT RECTIFIERS 



CH. XIII] 



In order to get good projection when this current supply only is 
available, a motor-generator set can of course be used, that is, the 
2 5 cycle current is used as power to drive a direct current dynamo 
( 682). The 25 cycle current can be changed to direct current by 
the use of a rectifier ( 683). Such current would of course be 
pulsating although always in the same direction. As the authors 
have never seen an arc supplied from a rectifier on 25 cycle current 
we can rrake no recommendation except to examine one of these 
machines in actual operation. If the arc should prove sufficiently 




FlG. 263. OSCILLOGRAMS OF THE POTENTIAL DIFFERENCE BETWEEN THE 

ANODE AND CATHODE. IN RELATION TO THE IMPRESSED ELECTROMOTIVE 

FORCE. 

(Cut loaned by the General Electric Co.; made from the original photograph). 

Curve A Potential difference between anode and cathode. 

Note that during half of the wave this difference is equal to the full impressed 
(line) voltage while during the other half wave the potential difference increases 
until the voltage has reached the constant value of 14 volts. When this occurs 
current is caused to flow through the arc and is used on the direct current side 
of the rectifier. 

Curve B Impressed electromotive force, i. e., instantaneous value of the 
line voltage. 



CH. XIII] 



CURRENT RECTIFIERS 



495 



free from flicker the rectifier would doubtless answer perfectly in 
all other particulars. There is no doubt about the motor-genera- 
tor; it will give perfect direct current for projection. 

685. Need of apparatus designed for the frequency used. 

All alternating current apparatus is designed to work with one 
frequency only, that is a transformer, for example, if designed for 
use on 60 cycle current will not work satisfactorily on either 135 
or 25 cycles. Hence, in ordering apparatus for alternating current 
it is necessary to ascertain and specify the frequency as well as the 
voltage and other particulars of the supply. This information 
can be furnished by the power company. 




FlG. 264. OSCILLOGRAMS OF THE ANODE CURRENTS. 

(Cut loaned by the General Electric Co.; made from the original photograph). 

Curve A Portion of the current furnished by one anode. 

Curve B Portion of the current furnished by the other anode. 

Note that from a single anode, current flows in one direction only, the mer- 
cury arc acting as a valve which prevents the current from flowing in the 
opposite direction. When current ceases in one anode the other anode com- 
mences to furnish the current. 



4Q6 WIRING FOR AN ELECTRIC CURRENT [Cn. XIII 

WIRING FOR AN ELECTRIC CIRCUIT FROM THE DYNAMO BACK TO 

THE DYNAMO 

686. For the purposes of projection by the aid of an arc lamp, 
the electric current required, whether it be direct current or 
alternating current, is practically always furnished by a dynamo. 
To make the electricity available there is a conductor of some kind, 
usually a copper wire extending from one pole of the dynamo to the 
arc lamp or lamps, and from them back to the other pole of the 
dynamo. Such a loop of wire from pole to pole of the dynamo 
forms an electric circuit, regardless of the length of the wire. With 
direct current, any part of the wire nearer the positive pole of the 
dynamo is positive to any part of the wire nearer the negative pole 
of the dynamo, hence the wire extending out from the positive pole 
of the dynamo is often designated the positive wire, and the wire 
received into the negative pole of the dynamo is called the negative 
wire. It will be seen from fig. 275, 280 that the circuit is a loop of 
wire with the two ends connected with the two poles of the dynamo. 

With alternating current, as stated above, there is no constant 
polarity, hence it is not correct to speak of negative and positive 
wires or positive and negative poles of the alternating current 
dynamo. 

687. Amperage for different purposes. As the quantity of 

electricity needed for different 
purposes varies, the capacity of 
the generator or dynamo must 
vary. Also the carrying capac- 
R ^/ ity of conducting wires is in genr 

eral proportional to their size, 
FIG. 265. SHORT CIRCUIT. hen(;e for , arge mmnts it is 

G ,c G Snd a uSo,ex d ,Sg across the ^cessary to have larger wires 

circuit making the path back to the than for small currents (see the 

dynamo (G) shorter than the course , i i -i i e < \ 

through the arc lamp (A) and the rheo- table below 694). 

If a wire is put across the points s 688. Short circuit. By a 

and c the electricity will take that path s h O rt circuit is meant the short- 

mstead of the longer path through the ,. , . , , 

arc. enmg of the distance which the 




CH. XIII] WIRING FOR AN ELECTRIC CURRENT 497 

current must travel to get back to the dynamo. In figure 265 if a 
wire were put across the circuit at the points s. c. instead of the 
current extending entirely around the circuit, it would take the 
shorter course. Short circuits are undesirable for two reasons : 
(i) the current is not available where wanted; (2) it may be 
dangerous. 

689. Ground. With many electric circuits such as with 
street railway circuits, one terminal of the dynamo is permanently 
connected with the earth. If now the wire connected to the other 




FIG. 266. AN ELECTRIC CIRCUIT WITH A SINGLE GROUND. 
C D The two poles of the dynamo. 
G Generator (dynamo). 

B I A conductor extending from the electric circuit to the ground (g 1 ). 
If all the rest of the circuit is insulated this will do no harm, but see fig. 267. 
g 1 The earth into which the conductor, B , extends. 
A Arc lamp. 
R Rheostat. 

terminal of the dynamo should also become connected with the 
earth, as through a water or a gas pipe, current would wholly or in 
part take that path back to the dynamo. 

When any part of the circuit is connected with the earth it is 
called a "ground." 

In case the dynamo and circuit are entirely insulated from the 
earth, a single ground will result in no flow of current outside the 
wire. If, however, two points in a circuit are connected to the 
earth the effect will be the same as if the two points of the circuit 
were connected to each other, by an additional wire (fig. 266, 267). 

690. Insulation of wires. To avoid short circuits and the 
consequent danger to men and animals and also the danger from 



498 



REGULATIONS FOR WIRING 



[Cn. XIII 



fire by the wires coming in contact with inflammable material, the 
wires are carefully insulated so that the current is kept in the circuit 
and not allowed to escape by taking short cuts or by going to the 
ground. Two things are necessary for this: (i) The naked wires 
must in no case touch each other at any point, for that would make 
a short circuit. (2) The naked wires must not touch anything 
which is a conductor. 

The wires are insulated by covering them with a coating of 
rubber, asbestos, silk, etc., that is, some substance which will 




FIG. 267. AN ELECTRIC CIRCUIT WITH A DOUBLE GROUND. 
C D The two poles of the dynamo. 
G Generator (dynamo). 
B 1 A conductor extending from the circuit near the pole C to the ground 

<'). 

B s A conductor near the pole D extending to the ground (g 2 ). 

In this case the current will short-circuit, passing from the point B 1 to g l and 
from g 1 to g', B 2 and back to the dynamo at the pole D instead of passing 
through the arc lamp (A ) and the rheostat (R). The single ground is dangerous 
only in that there is liable to be formed a second ground from some other part 
of the circuit. 

g 1 , g 2 The earth into which the conductors, B 1 , B 3 extend. 

A Arc lamp. 

R Rheostat. 

not serve as a conductor. Where the wire must be uncovered, as 
at switches, etc., some solid substance like porcelain, slate, hard 
rubber, glass or some other non-conducting substance is used, for 
the naked wires to rest against. 

REGULATIONS FOR WIRING: PRECAUTIONS 
691. National Electric Code. To make the wiring and con- 
nections of electric apparatus good and safe in every respect, the 
electrical engineers, architects and fire underwriters have formu- 



CH. XIII] REGULATIONS FOR WIRING 499 

lated definite rules for wiring, insulation and the character and 
construction of fittings, the installation of apparatus and of light- 
ing plants, etc. This national code of rules, with all authorized 
modifications found desirable from time to time, is published in 
pamphlet form by the National Board of Fire Underwriters for the 
guidance of those having electric wiring to do and apparatus to 
install. This board also publishes a list of electric apparatus and 
fittings which conform to this code. The two pamphlets can be 
secured by any one interested by sending five cents in stamps to 
cover postage, to the National Board of Fire Underwriters, 135 
William St., New York City, N. Y. 

General precautions : In wiring or changing wires and in work- 
ing about the arc lamp, rheostat, etc., the current should always be 
turned off at a switch which will render all the wires and apparatus 
to be changed in any way entirely without voltage ("dead"), so 
that no matter what is done there is no danger of receiving a shock 
or of short-circuiting. 

If "live wires" must be worked with, use the asbestos-patch 
gloves, and wrap the naked wires in asbestos paper so that it will 
be impossible to bring naked wires in contact. Remember also 
that a concrete floor, if at all moist, makes an excellent "ground" 
for the wires, and if a person stands on the moist floor with the 
wires in his hands the current is liable to pass through his body to 
the ground. It is safer to use a dry board or rubber mat on the 
concrete floor to stand on, or to wear rubbers. 

692. Municipal regulations for wiring, etc. In addition to 
the regulations of the National Board of Fire Underwriters, it 
frequently happens that there are special regulations by the 
municipality concerning the number and character of the general 
lights in a theater, etc., and also the source of the electricity for the 
arc lamp and for the general lights. There may also be special 
regulations for the number and color of exit lights and the source 
of the current for supplying them. It is necessary then to know, 
not only the latest regulations of the National Fire Underwriters, 
but the regulations of the city or state where the electric plant is 
installed. 



Soo 



INSTALLATION OF ARC LAMPS 



[Cn. XIII 



INSTALLATION OF AN ARC LAMP FOR PROJECTION 

693. In the first place it is necessary to know the maximum 
amperage to be used for the projection. The wiring, the fuses and 
the ballast (rheostat, inductor, etc.) must be adapted to this 
maximum current. 

If the installation is adequate for the highest current that may 
need to be used, it will of course be adequate for any smaller 
current. 

For drawing, and much of the ordinary magic lantern work, the 
current varies from 5 to 15 amperes, and if the installation were 
for such work alone, wiring and accessory apparatus which is safe 
for 15 amperes would suffice. If, on the other hand, the line were 
to be used for large halls also, and especially for opaque projection 
(Ch. VII), then the wiring and accessory apparatus would reed to 
have a maximum capacity of 50 to 60 amperes. For moving pic- 
tures, the line should safely carry a maximum of 75 amperes, or 
finally if kinemacolor moving pictures are to be shown in a large 
hall, the wiring and accessory apparatus must be adapted for an 
amperage of 100 to 200. 

The size of solid wires for different currents is given in the follow- 
ing table : 

694. Table of allowable carrying capacity of single copper 
wires of 98% conductivity.* 

AMERICAN INSTITUTE OF ELECTRICAL ENGINEERS 



No. Brown and 
Sharp Gauge 


Diameter in 
Millimeters 


Diameter in 
inches 


Circular 
Mils 


With Rubber 
Insulation 
Amperes 


With other 
Insulation 
Amperes 


No. I 


7.248 


.289 


83,690 


107 


156 


No. 2 


6-543 


257 


66,370 


90 


131 


No. 3 


5.826 


.229 


52,630 


76 


no 


No. 4 


5.189 


.204 


41,740 


65 


92 


No. 5 


4.620 


.182 


33,100 


54 


77 


No. 6 


4-U5 


.162 


26,250 


46 


65 


No. 8 


3.264 


.128 


16,510 


33 


46 


No. 10 


2.588 


.IO2 


10,380 


24 


32 


No. 12 


2.053 


.O8l 


6,530 


17 


23 


No. 14 


1.627 


.064 


4,107 


12 


16 


No. 16 


1.291 


.051 


2,583 


6 


8 


No. 18 


I.O24 


.040 


1,624 


3 


5 



*From the 1913 National Electrical Code, 18, pp 32-33. 



CH. XIII] 



INSTALLATION OF ARC LAMPS 



501 



694a. The carrying capacity of the different wires in this table is the 
amperage which can be safely and continuously carried by the wires without 
injury to the insulation or to the wire. The rubber covered wire is capable of 
carrying as great an amperage as the wires with more resistant insulation, but 
the amperage given, is that which experience has shown can be carried without 
undue injury to the rubber insulation, and with entire safety in continuous use. 

Furthermore, it should be said that the carrying capacity given in the table 
is by no means the maximum capacity which the wire could carry. For exam- 
ple, one might send a current of 20 amperes through a No. 18 wire, but this 
would soon injure the insulation from the overheating. By following the 
Electrical Code, one is on the safe side. 



695. Table of allowable carrying capacity of 
cables and cords composed of several small wires. 



flexible 



B & S Gauge 

No. of Wire 


Number of 
wires 


Rubber Insulation 
Amperes 


No. 18 
No. 1 8 
No. 18 


7 
19 
61 


25 
50 
1 2O 


No. 1 6 
No. 1 6 
No. 16 


7 
19 
61 


35 
70 
170 


No. 14 
No. 14 


61 
91 


235 
320 



ESTIMATED CARRYING CAPACITY 



No. 32 
No. 32 


40 
80 


5 

10 


No. 30 

No. 30 


15 
30 


3 
6 



1913 National Electrical Code, 94, p. 186-187. 



695a. This estimate is based upon the law that "The conductivity of a 
wire is directly proportional to its sectional area." Thus, No. 30 wire has a 
diameter of .01003 m - an d an area in circular mils of 010.03 x 010.03 = 100.6. 
The area in circular mils of No. 1 8 wire is 1624 The allowable carrying capa- 
city of No. 1 8 wire is three amperes when there is rubber insulation (see table 
above). Assuming the same proportional carrying capacity for the No. 30 wire 

then its capacity would be l624 = IO0 ' 6 , whence I624X =301.8 andX = .18 

3 X 

amp. If one small wire can carry .18 ampere, 15 should carry .18 x 15 = 2.7 
amperes or in round numbers, 3 amperes. If both cords are united into one 
conductor there would be 30 small wires with the capacity of .18 x 30 = 5.4 
amps, or 6 amperes in round numbers. 

For No. 32 wire in the same way: Thus, No. 32 wire has a diameter of 
.00795 in. The circular mils = 7.95 x 7.95 = 63.21 for each wire. 



502 INSTALLATION OF ARC LAMPS [Cn. XIII 

696. Selection of material for installing the arc lamp. After 
determining the maximum amount of current needed for the arc 
lamp, then the wire of proper size and quality and insulation to 
conform with the National Electrical Code should be obtained. 
The simplest way to do this is to go to some reliable dealer in elec- 
trical supplies and get the standard material.' 

Standard switches, etc., are all marked plainly so that there is no 
difficulty in selecting the correct sizes. In America, wire is more 
often designated by some standard wire gauge, e. g., that of Brown 
& Sharp, than by the actual diameter in millimeters or inches. In 
the above table the sizes in millimeters and inches corresponding 
with the B & S gauge numbers are given, also the area measured in 
circular mils. 

One must not forget that everything that is used wears out, and 
when any piece of apparatus or the wire becomes deteriorated by 
use it should be replaced. 

WIRING FOR THE ARC LAMP, THE RHEOSTAT OR OTHER BALANCING 
DEVICE, AND THE LAMP SWITCH 

697. Connection with the electric supply. It is assumed that 
the electric supply has been properly installed by an electric com- 
pany, or from a private dynamo, to within a short distance of the 
arc lamp. This supply will be in a proper outlet box, with fuses 
and switches in accordance with the National Electrical Code. In 
case the outlet box is on the wall close to the arc lamp, the simplest 
and most convenient connection between the lamp switch and the 
supply in the outlet box is by means of a separable attachment of 
the proper capacity for the maximum current. (See the table of 
flexible cables, 695.) If the current is direct, then it is a conve- 
nience to have this attachment irreversible, or polarized so that 

For No. 18 wire, as before, the circular mils are 1624 and the relative carrying 

capacity is assumed to be = ^' 2I , whence X = .116 amperes. If there 

3 X 

are 40 wires in each cord then each cord should carry .1 16 x 40 = 4.64 amperes, 
or in round numbers 5 amperes. If the double cord were used for each conduc- 
tor to the lamp, then in like manner twice as much could be carried, as there 
are 80 wires: .116 x 80 = 9.28 amperes or 10 amperes in round numbers. 



CH. XIII] 



INSTALLATION OF ARC LAMPS 



503 



one cannot make a wrong connection (fig. 268A). Such an attach- 
ment would also serve for alternating current, but is unnecessary, 
as it makes no difference which way the attachment is connected. 

The conductor from the electric supply in the outlet box to the 
lamp switch, if the distance is small, not over 2 to 3 meters (6 to 10 
ft.), is most conveniently made of flexible cable of the proper 
carrying capacity (see the table of carrying capacity of flexible 





FIG. 268. SEPARABLE WALL RECEPTACLES, POLARIZED (A) AND 

NON-POLARIZED (B). 

(Cuts loaned by H. Hubbell, Inc.). 

With direct current, a polarized attachment insures the same polarity with- 
out attention on the part of the operator; with the non-polarized form there 
is liability of reversing the polarity unless the connections are specially marked, 
and care is taken in putting the separable cap in position. Either form can be 
used with alternating current also. 

electric cables). The two wires or cables are often enclosed in a 
common sheath. 

698. In connecting the two wires to the attachment cap, the 
insulation is removed for a short distance (i to 2 cm. y in.), the 
wires scraped clean, twisted all together, and then turned to a loop 
to surround the set screw. Great care must be taken to avoid 
leaving any of the strands free; this would lessen the carrying 
capacity, but more important still, they might become displaced 
and make a short circuit (688). 



INSTALLATION OF ARC LAMPS 



[CH. XIII 



The wire is fixed firmly under the set screw, and if the current is 
to be large, 30 amperes and more, the wire should be soldered to its 
connection after the screw is firmly set down. 

699. Connecting the conductors to the switch. This is done 
exactly as for the attachment cap. 

In case direct current is used it is important to know which is the 
positive and which the negative wire. This should be determined 
before clamping the wire to the switch. The best method is by the 






FIG. 269. SEPARABLE ATTACHMENTS, POLARIZED (A) AND NON-POLARIZED 

(BC). 

(Cuts loaned by H. Hubbell. Inc.). 

The attachments A and B are for the ordinary bulb socket. 

A is polarized so that the same polarity of the wires is insured, for the connec- 
tion cannot be reversed. 

B is non-polarized and the polarity may be reversed every time the connec- 
tion is made. 

C is for receiving an incandescent lamp ; connection is made with the supply 
by inserting the prongs into an attachment plug which has been screwed into a 
lamp socket. 

use of the arc lamp ( 702), after the arc lamp and rheostat have 
been properly connected. 

700. Wiring the arc lamp, including the rheostat or other 
balancing device. From one pole of the switch (fig. 270), a wire 
of the proper size and insulation is carried directly to the 
negative binding post of the lamp, i. e., to the post for the lower 
carbon. From the other pole of the switch a suitable wire is 
carried to one binding post of the rheostat. From the other bind- 



CH. XIII] 



INSTALLATION OF ARC LAMPS 



505 




FIG. 270. WIRING OF THE ARC LAMP FOR PROJECTION. 
For full explanation, see fig. 3 and fig. 40. 

ing post of the rheostat a suitable wire is carried to the positive 
binding post of the arc lamp, that is to the binding post for the 
upper carbon. This puts the rheostat, or other balancing device 
in one wire, or in series, not in parallel, or across both the wires of 
the circuit. 

In securing the ends of the wires to the binding posts, scrape 
them, and twist the strands, then make a loop and put under the 
binding screw of the switch as described for the attachment cap. 
Usually for the rheostat, and the arc lamp, the wires are twisted 
and kept straight, then inserted into a hole, and a set screw turned 
down upon them. 

If flexible cord or cables are used for these connections, the wires 
on the end, after being scraped clean should be twisted and 
soldered, then none of the strands will escape to lessen the carrying 
capacity, or possibly to make a short circuit. 



5o6 POLARITY TESTS [Cn. XIII 

DETERMINING THE POLARITY WITH DIRECT CURRENT 

701. General statement and precautions. With direct 
current it is necessary, in most cases, to install the apparatus, like 
the ammeter, the voltmeter, the lamp, etc., in a very definite man- 
ner so that the current extends through the instrument in a given 
direction. That is, the positive end of the wire must be attached 
to the positive binding post. But when ready to install any piece 
of apparatus with direct current one rarely knows which is the 
positive and which the negative wire. It is necessary to find out 
by experiment. 

Precautions in making polarity tests. If possible, have a rheo- 
stat in the circuit before making the tests. One of the small 
rheostats for use with the small current arc lamp can be very easily 
introduced into the circuit (see fig. 188, 270 for wiring). If an 
adjustable rheostat is already in the circuit, set it for the least 
current. 

In making the tests never allow two naked wires to come in 
contact for that would complete the circuit and might burn out a 
fuse or do something worse. 

Never use a piece of metal, or a metal dish for holding the testing 
materials. Always use glass, porcelain or wood or some other 
non-conducting material. The tests are perfectly definite and safe 
if applied with due care. 

Remember also that when repair work on the line is done, the 
polarity of the supply wires may be changed. This would of course 
change the polarity of the arc lamp and a good light could not be 
obtained. One must be on the lookout for every possible trouble 
and have the knowledge and the resourcefulness to make the neces- 
sary modifications. 

DETERMINING THE POLARITY WITH AN ARC LAMP, WITH A 
VOLTMETER OR AN AMMETER 

702. (A) If an arc lamp and rheostat are available the 
simplest test is to connect the arc lamp, large or small, and rheostat 
as directed above ( 700). With proper carbons in place turn on 



CH. XIII] 



POLARITY TESTS 



507 



the current and strike the arc. After the lamp has burned a 
minute or two open the switch or pull the separable plug apart and 
watch the ends of the carbons. The one that remains red-hot the 
longer is the positive one, and the wire leading to it is the positive 




B 



FIG. 271. 



SIDE AND FRONT VIEWS OF THE RIGHT-ANGLE CARBON ARC WITH 
CORRECT AND INCORRECT POLARITY. 



A The upper figures show the correct polarity, that is, with the positive 
crater on the upper carbon. 

B The lower figures show reversed polarity, that is, with the lower carbon 
positive and hence the large crater on it. 

The photographs were made with a color screen in order to bring out the posi- 
tive and the negative craters with the greatest clearness. The exposure for 
the craters was instantaneous, then there was an additional exposure of 90 
seconds without a color screen, and with an illumination from a mazda lamp 
to bring out the carbons and give the appearance seen by the human eye (see 
also fig. 292-293). 



So8 POLARITY TESTS [CH. XIII 

wire. The method in practice is to watch the burning carbons 
through smoked glass or smoky mica. The positive one is markedly 
brighter than the negative one (fig. 271). 

If the upper carbon is positive the lamp is correctly installed, if 
the lower carbon is positive then it is improperly installed for 
ordinary projection. If the positive wire goes to the lower carbon, 
turn off the light by opening the switch or pulling the separable 
plug apart. Now reverse the position of the wires in the binding 
posts of the lamp, and this will bring the positive wire in connection 
with the upper carbon, and the negative wire in connection with the 
lower carbon (fig. 2, 270). 

If a non-polarized separable plug is used (fig. 268 B), the simplest 
way to reverse the polarity is to pull the cap off, turn it half way 
round and insert it again. When found to be in the correct posi- 
tion mark the socket, the plug and the cap in some way so that the 
connections can be made at some future time with certainty. 
There are polarized plugs (fig. 268A) in which the connections are 
so arranged that the attachment plug can be inserted only in one 
way, thus avoiding the change of polarity when once the wiring is 
correctly installed. 

When the polarity is found to be correct it is advantageous for 
future work to mark the insulation of the positive wire near the 
switch with red paint. The positive side of the table switch 
should also be marked with a + sign or with P. using black or red 
paint. In like manner the insulation of the wire near where it is 
connected with the binding post of the arc lamp should be marked 
red, and a + or P. should be put alongside the binding post for the 
upper carbon unless it is so evident that no mistake is likely to 
occur. 

(B) Testing the polarity with a direct current voltmeter 
To do this connect the voltmeter with both wires (fig. 272). 
Turn on the current by closing the switch and if the positive wire 
is connected with the positive binding post the voltmeter will 
record the voltage in the line. If the wires are wrongly connected 
then the hand will try to move off the dial face below zero. If the 
hand does not register the voltage, open the switch, and reverse the 



CH. XIII] 



POLARITY TESTS 



509 



position of the wires in the binding posts of the voltmeter. Turn 
on the current and the voltmeter will register. It is well to mark 
the insulation of the positive wire with red, or in some .other way. 




FIG. 272. VOLTMETER FOR TESTING POLARITY. 

G Dynamo for direct current. The positive pole is above and the negative 
pole is below, as indicated by the arrows. 

Vm The terminals of the voltmeter are correctly connected across the line 
(in multiple) or to both wires and the hand indicates the voltage on the dial. 
If the terminals were wrongly connected the hand would not register. 

A Arc lamp. 

R Rheostat. 

The arrows indicate the direction of the current. 

The -}- and signs indicate that any point in the circuit nearer the positive 
pole of the dynamo is positive to any point nearer the negative pole. 




FIG. 273. AMMETER FOR TESTING POLARITY. 

G Dynamo for direct current. The positive pole is above and the negative 
pole below. 

Am Direct current ammeter. The terminals a +, b are connected along 
one wire (in series). If the positive pole of the ammeter is connected to the 
circuit next the positive pole of the dynamo, and the negative terminal in the 
wire toward the negative pole of the dynamo, as here shown, the hand will 
register when there is current flowing. If the connections are reversed the 
hand will not register when the current is flowing. 

a+, b The positive and the negative terminals of the ammeter. 

A Arc lamp. 

c+ The positive carbon. 

c The negative carbon (the minus sign is put parallel with the carbon to 
show the direction of the current). 

R Rheostat. 

The + and signs and the arrows are as with the voltmeter (fig. 272). 



5io POLARITY TESTS [Cn. XIII 

(C) Testing the polarity with a direct current ammeter 
The circuit should be connected with a rheostat and an arc lamp 
or one or more incandescent lamps in series (along one wire) then 
the switch is opened and the ammeter is inserted in one wire (in 
series), (fig. 273). Now turn on the current and light the lamp 
( 30). If the wires are correctly connected the ammeter will 
indicate the amount of current flowing; if it is wrongly connected 
then the hand will try to move off the dial below zero. That is, the 
positive wire has been inserted in the negative binding post of the 
ammeter, and the negative wire in the positive binding post. 
Open the switch, and reverse the position of the wires in the binding 
posts ; turn on the current and the hand will register the amperage. 
The positive wire can then be marked red or in some other way. 

CHEMICAL POLARITY INDICATORS 

703. Litmus, iodized starch, salt solution and potato indica- 
tors. (A) Litmus indicator. Take some blue litmus or other 
acid-alkaline testing paper, about 10 cm. (4 in.) long and place it 
on a pane of glass or a porcelain plate. Moisten it well. Separate 
the ends of the wires as indicated in the testing lamp (fig. 21). 
Put the two ends about 10 centimeters (4 in.) apart on the mois- 
tened litmus paper. Turn on the current. The positive wire will 
turn the blue litmus paper red when the current flows. Turn 
off the current and mark the positive conductor red, or white. 

(B) Iodized starch polarity indicator. Make some starch paste 
by mixing 15 grams (y oz.) of dry starch (corn starch, laundry 
starch or wheat flour) with 300 cc. (10 oz.) of cold water. Add y 
gram (7 or 8 grains) of iodide of potassium. Now heat the mixture 
with constant stirring until the starch is cooked. Put some of the 
iodized paste in a glass or porcelain dish and insert the separated 
wires to be tested in the paste. Turn on the current and the starch 
at the positive pole will be turned blue. Turn off the current and 
mark the positive wire in some way. (The iodized starch test is 

702a. If one uses a voltmeter or an ammeter of the new, soft-core type 
(Eclipse Volt and Ammeters) which register both alternating and direct cur- 
rent, one cannot determine polarity with them, for they register whichever 
way they are connected with the circuit. 



CH. XIII] 



POLARITY TESTS 



the one commonly employed for weak currents for it is very 
sensitive; it is, however, equally good for large currents). 

(C) Salt and water polarity indicator. Make a y% solution 
of common salt (NaCl) in water. Place the solution in a glass or 
porcelain dish about 10 cm. (4 in.) across. Insert the two separ- 
ated wires to be tested in the liquid and turn on the current. When 
the current is on, many small bubbles will appear at the negative 
pole. In making this test remember the precautions ( 701). 

(D) Raw potato polarity indicator. Cut an ordinary uncooked 
potato in half. Insert the wires into the potato having the wires 
as far apart as possible. Turn on the current. The potato 
around the positive pole will turn greenish. If the potato is quite 




FIG. 274. THREE-WIRE ARC LAMP OF THE BAUSCH & LOME OPTICAL COMPANY 
For a full explanation see fig. 14.5 and 704. 



512 WIRING FOR ALTERNATING CURRENT [Cn. XIII 

moist, many small bubbles will appear around the negative pole. 
But the greenish color given at the positive pole is the most certain. 
Turn off the current and mark the positive wire red. 

With the other chemical tests (A, B, C) the indications are in no 
way dependent on the metal forming the wire, but with the potato 
test the poles entering the potato must be copper or contain 
copper. 

704. Wiring the three-wire automatic lamp of the Bausch & 
Lomb Optical Company. This lamp is regulated wholly by elec- 
tricity, there being no clock-work. In wiring the lamp one pro- 
ceeds exactly as described above ( 693-700), except that a wire 
is carried from the positive side of the switch to the middle binding 
post of the lamp directly. Another wire from the same point is 
carried down to the resistor or rheostat, and from the rheostat a 
wire to the positive or upper binding post of the lamp. From the 
negative pole of the switch a wire is. carried directly to the lower 
or negative binding post of the lamp. This wiring gives the full 
voltage of the line for the electric mechanism governing the lamp 
(see fig. 145). 

WIRING FOR ALTERNATING CURRENT 

705. This is precisely as for direct current, and one does not 
have any trouble about the polarity. It makes no difference 
which supply wire is connected with the upper carbon and which 
with the lower. 

706. Insertion of the rheostat, inductor or other balancing 
device. It makes no difference in which of the lead wires the 
rheostat, etc., are inserted. Just as with direct current, however, 
the balancing device must be inserted along one wire (fig. 281), 
otherwise the current would not traverse the entire circuit. 

707. Position of the rheostat, etc. The balancing effect of 
the rheostat is the same no matter where it is installed in the special 
circuit for the arc lamp. For convenience it is frequently put on 
or near the projection table. This is especially necessary if the 
rheostat is adjustable. With a fixed rheostat it is sometimes safer 



CH. XIII] WIRING FOR ALTERNATING CURRENT 513 

to put it near the supply intake, especially if that is at a consider- 
able distance from the lantern or other projection apparatus, then 
in case of a short circuit in working about the lamp or table switch, 
an excessive current could not flow, and there would be much less 
danger from fire or the burning out of fuses. (See also 708). 

708. Wiring when the arc lamp is far from the supply. 

When the supply is at a considerable distance from the arc lamp 
the flexible wire connection is sometimes used for temporary work, 
but is not suitable for permanent installation. 

Instead of a conduit, well insulated wires are sometimes used 
from the general supply box to the neighborhood of the arc lamp. 
The wires must be secured by porcelain or other non-conducting 
supports every meter (3 or 4 feet) which will separate them from 
the wall i to 2 cm. ($4 in.) and from each other 5 to 7 cm. (2^/2 in.) 
and hold them in place. Where the wires pass through partitions, 
each wire should have its own porcelain tube so that is does not 
come in contact with the partition. The safe rule in wiring is to 
treat the rubber covered wires as if they were naked. At the end 
it is desirable to have a metal box for the special fuse block and 
switch. An attachment fixture is also very convenient (fig. 270). 

For the position of the rheostat, etc"., see 707. 

709. Wiring an arc lamp for large currents. Arc lamps for 
opaque projection (Ch. VII) and for moving pictures (Ch. XI) 
require large amperages, and frequently the lamps become very 
hot, especially if the lamp-house is not large and well ventilated. 
For lamps requiring the large currents it is best to use flexible 
cables of higher capacity than is needed outside the lamp-house. 
The wire should also be insulated with some fire-proof material like 
woven asbestos. 

The ordinary, rubber insulation will answer for low amperages 
especially when the lamp-house is well ventilated. An excellent 
wiring material is the flexible cord used for heating apparatus. 
This has rubber insulation, and also woven asbestos, and the 
outside is covered with, woven cotton to protect the asbestos. Of 
course a flexible cord of the proper carrying capacity should be 
selected. 



SU SWITCHES, FUSES, CIRCUIT BREAKERS [Cn. XIII 

If it is difficult to get double cord of the right size, then each of 
the wires to the lamp can be composed of the double cable. This 
is easily done by removing the insulation at each end of the double 
cord and twisting both the wires together. (See the tables 694, 
695, for the carrying capacity of flexible cord and cables). 

710. Wiring the arc lamp with a three-wire supply. Only 
two wires go to the arc lamp, then if one must connect the arc lamp 
for projection to a three-wire supply system it is necessary to 
remember that the middle (neutral) wire and either outer wire will 
give 1 10 volts the same as the two-wire no volt circuit. 

If connection is made with the two outer wires then 220 volts 
will be used in the arc lamp. In this case a rheostat for a 220 volt 
circuit must be employed, or two no volt rheostats in series (fig. 
287). 

Naturally one would connect with the middle or neutral and an 
outside wire and employ the usual no volt rheostat but for the 
fact that such an arrangement would badly unbalance the work of 
the line, and might cause trouble if the electric circuit was running 
nearly on full load. It is therefore safer to connect with the out- 
side wires and use the requisite amount of ballast. 

SWITCHES, CIRCUIT BREAKERS AND FUSES; THEIR CHARACTER, 

INSTALLATION AND USE 

711. A switch is a device by means of which a gap (fig. 275 
and 276) can be made in an electric circuit thus stopping the flow 

of current. 

A switch should be so con- 
structed that when it is opened 
\ it makes a gap in all the wires 
B of the circuit. For example, in 
J a two-wire circuit, the switch 
should make a gap in both wires, 

FIG. 275. CIRCUIT WITH A BREAK and in a three-wire circuit, a gap 
OR GAP. 11 .-, rf 

.. . it in all three wires. If such a 
Unless the metalhe circuit, irom the 

dynamo, G, back to the dynamo, is switch IS used the line beyond 
complete, no current will flow. A gap the sw i tc h is "dead, "and no CUr- 
m the circuit (B) prevents the now ot 
current. rent can be drawn from it. 



CH. XIII] SWITCHES, FUSES, CIRCUIT BREAKERS 



SIS 




LW 





FIG. 276. SNAP AND KNIFE SWITCHES SHOWING OPEN AND CLOSED CIRCUIT. 

A Snap switch with circuit closed (current on). 

B Knife switch with circuit closed (current on). 

C Snap switch with circuit open (current off). 

D Knife switch with circuit open (current off). 

A W Wires from the switch to the arc lamp. 

Base The insulating support of the knife switch. 

H Handle of the switch blades. 

L W Supply wires for the electric current to the switch. 

There are two main forms of switches: The knife switch like 
that shown in fig. 276 B, D, and the snap switch, which rotates 
(fig. 276 A, C). Any switch to be installed should conform in its 
construction with the National Electric Code and be plainly marked 
with its capacity -voltage and amperage and the maker's name. 

712. Installation of a switch. The non-combustible, non- 
conducting base should be fastened to some support, and then the 
wires of the line cut and scraped and connected firmly in the bind- 
ing posts or under the binding screws. If the current is over 30 
amperes the wires should also be soldered to the switch after the 
screws are well set down. A switch at the supply for the building 



SWITCHES, FUSES, CIRCUIT BREAKERS 



[CH. XIII 



or special plant should be enclosed in a metal box where it can be 
easily got at, but not where the naked metal parts might inad- 
vertently become short-circuited. 

It is necessary also to put the switch in such a position that 
when it is opened it will not close of itself by gravity. If the 
switch is in a vertical position it must be placed with the hinge 
below, so that gravity will tend to open it, never to close it (fig. 
277). 

If the switch is horizontal, then the hinge should be tight enough 
so that the blades will remain in any position in which they are 
placed. For a double-pole, double-throw switch for two lamps see 
fig. 162. 

A knife switch has an appreciable amount of naked metal 
exposed. It therefore makes a short circuit easily possible. For 
use with projection apparatus, especially if high amperages are to 
be used as with opaque projection and with moving pictures it is 




Base 




AW 



FIG. 277. OPEN KNIFE SWITCH IN A VERTICAL 

POSITION, WITH THE HANDLE BELOW so THAT 

THERE IS NO DANGER OF THE SWITCH 

CLOSING BY GRAVITY. 

L W Line wires from the electric supply (fig. 
270) to the switch. 

A W Arc lamp wires from the switch to 
the arc lamp. A rheostat is inserted in one of 
them (fig. 270). 

5 C Spring clamps pressing against the switch 
blades when the switch is closed, thus making 
good metallic contact. 

Base The insulating base of the switch. It 
is held in position by two or more screws. 

Hg Hinges of the switch blades. 

5 B Switch blades. When the switch is 
closed these blades make a continuous circuit, 
and when the switch is open the circuit is 
broken. 

Cb Cross-bar of insulating material to 
which the switch blades and the handle are 
attached. 

H Handle for opening and closing the switch. 
It is of insulating material 



CH. XIII] 



SWITCHES, FUSES, CIRCUIT BREAKERS 



517 



advantageous to enclose the switch in a metal box with a slit 
allowing the handle to project and move so that the switch can be 
opened and closed. As only the handle is exposed with this arrang- 
ment the operator is safe when manipulating the switch in the dark 
(fig. 278). See also 714. 

713. End of the switch to connect with the supply wires. 

Sometimes the supply wires are connected with the hinge end of the 
switch as in fig. 2. This has the disadvantage that the switch is 
then energized up to the break at the handle, when the main supply 
is on. As the switch is liable to get out of order and need screwing 
up occasionally it is better to insert the lead wires in the opposite or 




FIG. 278. ENCLOSED SWITCH IN A HORIZONTAL POSITION. 

Commencing at the right: 

L W Supply or line wires from the outlet box (fig. 270) to the table switch. 

k Key for locking the metal cover when it is closed. 

H Handle of the knife switch. It projects through the slot (5) in the cover. 
In the position shown the switch is open. 

sb Switch box. This is a sheet iron box enclosing the switch so that noth- 
ing can come in contact with the naked metal of the switch. Only the switch 
handle projects beyond the box. The enclosing box is represented as trans- 
parent in order to show the switch and its connecting wires within. The bot- 
tom of the enclosing box is covered with asbestos board and the switch base 
rests on the asbestos, not on the metal of the box. 

hg Hinge of the metal cover. By turning the cover over to the left the 
entire switch is exposed. 

A W Wires from the switch to the arc lamp. 



5i8 SWITCHES, FUSES, CIRCUIT BREAKERS [Cn. XIII 

jaw end of the switch as in fig. 277, then when the switch is open 
the hinges and blades are "dead" and can be put in order with 
safety. 

714. Snap Switches. These are sometimes used for turning 
on and off the current at the operating table. They are mounted 
on insulating material like porcelain, and are enclosed by a metal 
covering which is lined with insulating material. The key or 
button for turning on and off the current is also of insulating 
material. This form of a switch around the work table is con- 
venient, and avoids any danger of accidentally short-circuiting the 
line. It should turn on the current and turn it off with a snap. It 
is also desirable that there should be a sign indicating when the 
current is on or off, as one cannot see directly as with the knife 
switch. If such a switch is used, make sure that it is of the right 
capacity for the maximum current and that it conforms in every 
way with the standard requirements. It will be plainly marked so 
that after it is installed one can see at any time the current and 
voltage for which it is designed. Snap switches are better adapted 
for small currents, than for large ones. Knife switches are to be 
used on lines with large currents. 

FUSES AND CIRCUIT BREAKERS 

715. Fuses and circuit breakers are devices for opening or 
breaking the circuit whenever the current in any particular situa- 
tion becomes too great. For example, if a part of the line should 
be short-circuited. 

The devices used are of two kinds ; fuses, and magnetic cut-outs 
or circuit breakers. 

716. Circuit breakers. The circuit breaker is a device by 
which a magnetic trip releases a catch which allows a large switch 
to open, thus breaking the circuit. 

The great advantage of a circuit breaker is that nothing is 
burned out or melted. It is only necessary to close the switch 
again and the current will be on. It acts instantly whenever the 
current rises above the amperage for which it is adjusted. 



CH. XIII] SWITCHES, FUSES, CIRCUIT BREAKERS 519 

717. Fuses. A fuse is a wire of low melting point forming 
part of the circuit. If the current becomes too great this fuse is 
melted, thus making a gap in the line. The fuse is then said to 
burn out or to "blow." If the current becomes much too great as 
in a short circuit the fuse will "blow" instantly, if however, the 
current is only slightly larger than the fuse is designed for as for 
example, when striking the arc in an arc lamp the fuse will not 
"blow" instantly, and if the overload is only for a short time it will 
not melt at all. If the overload continues for some time, however, 
the fuse will get hotter and hotter until its melting point is reached, 
when it will melt and open the circuit. This property of the fuse is 
of great advantage when using arc lamps, for the temporary over- 
load in lighting the arc lamp is unavoidable. 

718. Location and installation of fuses. Like the switch, the 
fuses should be placed in the path of all the wires of a circuit i. e., 
with a two-wire system two fuses, and with a three-wire system, 
three fuses, etc. The wiring of a fuse block is the same as for a 
switch ( 712). 

There is always a switch in the supply box from the electric 
lighting system or from the private dynamo. In this box are also 
fuses to open the circuit in case of accidental short-circuiting. The 
fuse block, whether for cartridge fuses or for plug fuses should be 
selected with care to make sure that it is of the right capacity 
for the maximum current and conforms to the standard code. The 
fuses are plainly marked, so there need be no mistake. 

One should not use fuses of higher capacity than the line was 
designed for, for fear of fire or other accident. 

If the supply box is some distance from the arc lamp, many 
careful operators have fuses as well as a switch at the supplemen- 
tary supply box in the operating room, when a conduit or fixed 
wires are carried from the main supply to the operating room. The 
fuse nearest the arc lamp is preferably of somewhat less capacity 
than the ones farther away, then if a fuse is blown it will be the 
handiest one to renew. 

719. Fuses and the wattmeter. If but a single meter is used 
to measure the current for arc lights, house lights, heating appara- 



520 SWITCHES, FUSES, CIRCUIT BREAKERS [Cn. XIII 

tus, etc., then each group should be separately fused after the 
wattmeter, for then if one part of the line is cut out the rest can go 
on drawing current. For example, if the arc lamp were misman- 
aged it ought not to be possible to blow out the fuse for the house 
lights, and the reverse. 

720. Location of fuse blocks. The general rule is that there 
must be a fuse block wherever there is a change in the size of the 
wire used. These fuse blocks must be in cabinets in plain sight and 
readily accessible. Usually also, with every fuse block there is a 
knife switch. 

721. Capacity of fuses. The rated capacity of fuses should 
not exceed the allowable carrying capacity of the conducting wire 
(see tables 694, 695), and circuit breakers should not be set more 
than 30% above that allowable capacity. 

The allowable capacities for incandescent lamp lines are as 
follows : 

55 volts or less 12 amperes 

55-125 volts 6 amperes 

1 25-250 volts 3 amperes 

For electric lighting each special circuit or line should not be used 
for a current greater than will give a power of 660 watts. This 
would mean for example, that if one wished to use 60 watt lamps 
there could be only 1 1 of the lamps on a single line. If 40 watt 
lamps were used then there might be as many as 16 lamps on a line, 
etc. 

In using flat-irons and other heating devices on an electric lamp 
circuit, care must be exercised not to turn on any lights on that 
branch of the circuit. 

Likewise in using the small arc lamp for drawing with the micro- 
scope, ultra-microscopy, etc., where from four to six amperes of 
current are needed, one should not use incandescent lights on 
that line at the same time, for the current would exceed the allow- 
able amount and probably blow a fuse. 

722. Replacement of fuses. As fuses are liable to blow out 
it is well to have a supply on hand, then the burnt out ones can be 



CH. XIII] RHEOSTATS AND OTHER BALLAST 521 

quickly replaced. To replace a fuse, open the nearest switch which 
will turn off the current from the line. Take out both fuses, and 
examine them; only one is likely to have melted. It is usually easy 
to tell which. Discard that one, then insert two good fuses of the 
proper capacity, close the switch, and the current will be available 
again. 

If the lights on a particular line go out from the blowing of a fuse, 
and one is not sure which branch it is in the fuse box, the one is 
easily found by using the testing lamp (fig. 21) beyond the fuses. 
The lamp will light on all the lines with perfect fuses when put 
across the blades of the special line switch, or when put in contact 
with any naked metal part across the line. The line with a burned 
out fuse will not light the testing lamp, when it is applied beyond 
the fuse. 

RESISTORS OR RHEOSTATS: INSTALLATION AND USE 

723. Resistor or rheostat. A rheostat is a conductor having 
considerable resistance ; it is placed in an electric circuit to regulate 
the amount of current. In passing through the rheostat much heat 
is developed by the energy consumed in overcoming the resistance. 
This energy consumption is a dead loss. 

The conductor used is ordinarily in the form of wire or strips of 
metal such as German silver, iron or nickel. 

724. Amount of resistance needed. Electricians have 
worked out with much accuracy the resistance of different metals 
and by consulting the tables furnished in books on electrical 
engineering one can find how great a length of a given size iron 
or German silver wire is necessary to afford the proper resistance 
for any given constant voltage, as no or 220. See 



724a. Ohm's Law and its application to projection apparatus. While the 
units, volt, ampere and ohm ( 654-657) might be worth defining, still it would 
lead to no very practical results unless there was a definite relation between the 
electric quantities for which these units stand. 

It has been found by experiment that there is a very definite relationship, 
known as Ohm's Law. (For a history of the discovery of this law by Ohm, see 
Dr. Shedd in the Popular Science Monthly for Dec., 1913). 

Briefly stated Ohm's law is: "The current in a given circuit is directly pro- 
portional to the electromotive force, and inversely as the resistance:" 
Nichols, p. 294. 



522 RHEOSTATS AND OTHER BALLAST [Cn. XIII 

As stated by Norris it is: "The electromotive force consumed in the 
resistance of a conductor, is proportional to the current." P. 8. 

Using the terms now employed in place of electromotive force (voltage), 
resistance (ohmage) , and current (amperage) , the law can be stated thus : 

(1) The voltage in a conductor is equal to the amperage multiplied by the 
ohmage: V = A O. 

(2) The amperage is equal to the voltage divided by the ohmage: A = 

(3) The ohmage is equal to the voltage divided by the amperage: O = 

A 

As V = A O = i . From this form is derived the very simple dia- 

A O 

gram used practically in getting the formula for the value of any single quantity 
if two are known. The formula for the unknown quantity is found thus : 

Cover the letter representing the unknown 
V quantity, and the remaining letters will indicate 

the value of the unknown quantity. 

" Examples: 

FIG. 279. DIAGRAM OF i. If the voltage and amperage are known, 

OHM'S LAW FOR SOLV- wh at is the ohmage? 

ING PROBLEMS ( 7243). Cover the O and there remain V/A and this is 

V = Voltage equal to O, i. e., O = V/A. Suppose the volt- 

A _ . age is 1 10 and the amperage is 20, what is the 

O = Ohma e ohmage? Applying the formula, O = 1 10/20, 

2. If the voltage and the ohmage are known what is the amperage? Here 
if A is covered there is left V/O, whence the amperage equals the voltage divided 
by the ohmage. If the voltage is 220 and the ohmage is 5.5 as before, what is 
the amperage? A = 220/5.5 =40 amperes. This example also illustrates the 
fact that if the ohmage remains constant the amperage will increase in direct 
proportion to the voltage. (See Dr. Nichols' definition above). 

3. If the amperage and ohmage are known what is the voltage? Here the 
unknown quantity is represented by V. If this is covered there will be left 
A O, whence V = A O. If the amperage is 15, and the ohmage 8 then the 
voltage must be 15 x 8 = 120, i. e., V = 120 volts. 

As a further example suppose one wished to make a water-cooled rheostat 
(fig. 283) and he had some wire which had an ohmage or resistance of 0.25 ohm 
per meter, how much wire would be needed with a voltage of no and an 
amperage of 15? Here voltage and amperage are known. From the formula 
it is seen that ohmage equals voltage divided by amperage: whence 1 10/15 = 
7.33 ohms, the total resistance required. 

Now as 55 is the voltage required by the arc with the direct current arc 

lamp, the lamp itself must offer a resistance of . ~ f r 3-66 ohms. 

A = 15 

As the total ohmage needed is 7.33, the rheostat must possess the difference 
between 7.33 and 3.66 or 3.67 ohms. 

If each meter of the wire to be used offers a resistance of 0.25 ohm, it will 

require for 3.67 ohms, ^' ? = 14.68 meters of the wire for the rheostat. (For 

0.25 
the wattage of the current see 660). 



CH. XIII] RHEOSTATS AND OTHER BALLAST 523 

725. Getting rid of the heat developed. As much heat is 
developed in the rheostat, it is necessary to so arrange the coils of 
wire, etc., forming it, that the heat can easily escape, otherwise the 
wire might get so hot that it would melt. Provision for carrying 
away the heat then is of prime importance. For example, a large 
iron telegraph wire would get red hot in the air if it were used for 
100 amperes, while a much smaller wire if immersed in water would 
carry the current easily on account of the rapid dissipation of the 
heat in the water. 

Ordinarily the resistance wire is in coils, and these are hung on 
non-conductors in such a way that there is free circulation of air 
around and through the coils to carry off the heat. 

Sometimes the wire or strips of metal serving for the resistance 
are imbedded in porcelain, and a considerable surface of the porce- 
lain being exposed to the air, the heat readily escapes. This is 
often the method with the rheostats used for dimming the lights in 
theaters (theater dimmers). (See fig. 183, 186 for a theater 
dimmer used as a rheostat). 

In fig. 198 is a small rheostat with the metal in a helical coil and 
wound around a porcelain core. This rheostat is for the small arc 
lamp to be used on the house lighting system, and restricts the 
current to 4-6 amperes. 

726. Fixed rheostat. This is a rheostat in which the entire 
amount of resistance wire must be traversed whenever the current 
is on, the amperage of the current is then practically constant. 
For example in using the arc lamp if the rheostat is designed for 1 5 
amperes, that current must always be used. The fixed rheostat is 

best adapted for any place where 
many use the same apparatus 
(fig. 280). 

727. Adjustable rheostat. 
The adjustment consists of an 
arrangement by which a greater 

or less length of the resistance 
FIG. 280. CIRCUIT WITH DYNAMO (G) , . , j , ,, 

ARC LAMP (A), AND FIXED RHEO- Wlre can be included in the cir- 
STAT (R). cuit at will. The more resis- 




524 RHEOSTATS AND OTHER BALLAST [Cn. XIII 

tance in the circuit the less will be the amperage, and the less resis- 
tance the higher the amperage. 

In some forms it is possible to have a great range of current, say 
from 5 to 45 amperes (fig. 281); in other forms the range may be 
limited, say from 1 5-2 5 amperes. 

For the projection microscope and the magic lantern it is desir- 
able to have a rheostat giving a range of amperage from 5 to 25 




FIG. 281. THE USE OF AN ADJUSTABLE RHEOSTAT AS BALLAST FOR AN ARC 

LAMP 

G Generator (dynamo). 

A Arc lamp with right-angle carbons. 

AR Adjustable rheostat. 

5 If the movable contact-arm is at 5, the resistance allows but 5 amperes to 
flow. 

25 If the contact-arm is at 25 then only half of the resistance is in the cir- 
cuit and 25 amperes of current can flow. 

45 If the contact-arm is opposite 45, only a small amount of resistance is in 
the circuit and forty-five amperes of current is allowed to flow. 

The arrow indicates the direction to turn the contact-arm to increase the 
current. 



amperes. Such a rheostat is not difficult to construct, nor is it 
expensive. The theater dimmer shown in fig. 183 is of this range. 
But an adjustable rheostat requires judgment for its proper use; 
for apparatus to be used by everybody it is better to have a fixed 
rheostat ( 726). 

728. Installing the rheostat. It is usually placed close to the 
arc lamp, i. e., inside the lamp switch, so that when the lamp switch 
is open the current is entirely off the arc lamp and its rheostat. 



CH. XIII] RHEOSTATS AND OTHER BALLAST 525 

In wiring the rheostat, it is to be placed in one wire, (in series) as 
all the current must pass through it (fig. 188, 281). It makes no 
difference whether it is placed in the wire going to the upper carbon 
or coming from the lower carbon. 

729. Calibration of a rheostat. The maker of a rheostat 
should mark plainly upon it its capacity if it is of the fixed form. 
If it is adjustable, then the range of the rheostat should be given. 

Furthermore, the lower range should be plainly marked at the 
lowest step and the highest range at the highest step. The user of 
a rheostat like that in fig. 145 could not tell easily which way to 
turn the knob to increase or diminish the current unless the maker 
indicates the amperage at the two ends of the steps. In case there 
is no indication a person can determine it for himself if he has an 
ammeter. 

Insert the ammeter in one wire of the line (fig. 273). Turn the 
knob of the rheostat to the middle step, insert proper carbons in the 
arc lamp, and turn on the current. When the lamp is burning 
properly note the reading on the ammeter. Turn the knob toward 
one side and the ammeter will indicate whether there is more or less 
current. One can in this way find the amount of current delivered 
for the different positions. It is well to mark on the rheostat face 
with white paint the amperages corresponding to these positions. 
It is also a help to have an arrow pointing from the lowest to the 
highest amperage (fig. 182, 281). 

730. Home-made rheostats. While it is altogether false 
economy to use anything but the best in the form of a rheostat it 
is worth while knowing how one could be made in case of urgent 
need. 

731. Barrel or bucket type of salt water rheostat. A wooden 
bucket or barrel is used. In the bottom is placed a large plate of 
iron, and one end of the supply wire is firmly fixed to this. The 
other end of the wire is fixed to another mass of iron. The barrel 
or bucket is then filled nearly full of water, and enough common 
salt added to make about a ^4% solution. The water should be 
well stirred to evenly distribute the salt. The upper iron and 



526 



RHEOSTATS AND OTHER BALLAST 



[CH. XIII 



wire are then covered by a burlaps sac so that there can not be a 
metallic contact between the masses of metal. This upper wire 
and its iron are then immersed in the barrel. If now the arc lamp 
is fitted with carbons, and the switch closed the arc will form as 
usual, the salt water and the iron plates serving as a rheostat. 

wt ._ wi 



W2 




FIG. 282. SALT WATER RHEOSTAT. 

W lt W a Conductors. One end of conductor W, is connected to an iron 
plate P 3 in the bottom of the dish. The other end is connected to the plate P, 
which is suspended by a string wound around the clamp. The burlaps sack S, 
prevents contact of P f and P, with resulting short circuit should the upper 
plate be let down too far. It is safer still to have both plates covered, and 
the container must be of wood, glass or stoneware, i.e. some non-conductor. 

The jar contains a %% solution of salt. The resistance is regulated by 
raising or lowering the plate P,. If more current is required, lower the upper 
plate, if less current, raise P, so that the two plates will be farther apart. 



CH. XIII] 



RHEOSTATS AND OTHER BALLAST 



527 



If one wishes a greater amperage the upper wire is lowered in the 
barrel and if less current is desired the upper iron is lifted higher 
in the barrel (fig. 282). Of course there must be some means of 
holding the upper wire in position when it is at the right height 
in the barrel. 



Wi 



Wi 




FIG. 283. WATER COOLED RHEOSTAT. 

W t , W 2 Conductors. 

R Rheostat composed of the proper length of small naked wire wound 
around a frame of wood. The two ends of this resistance wire are soldered to 
the cut ends of the supply wire W 3 W a . The rheostat is then immersed in 
running water and the containing vessel of wood, glass or stoneware is placed in 
a sink. 



528 RHEOSTATS AND OTHER BALLAST [CH. XIII 

In no case should one use naked wires for this rheostat, but the 
rubber, water-proof insulated copper wires required by the National 
Electric Code. The ends of the wires must be scraped and fastened 
to the plates of iron. This is rather a poor make-shift for a rheo- 
stat. The water soon heats up, and as it heats the resistance 
becomes less so that more current flows. Then to counterbalance 
this, fresh cold water can be added or the upper plate lifted to make 
the distance between the iron plates greater. Furthermore, increas- 
ing the amount of salt lessens the resistance. If there is too much 
salt there will be too much current, if too little one cannot get 
enough current without bringing the iron plates very close together, 
and this is not safe. 

732. Home-made water cooled rheostat. A home-made 
rheostat can be constructed of small, naked wire of the proper 
length as shown by calculation or by the electrical tables. The 
wire is wound around a wooden frame in a single layer, care being 
taken that the different turns do not touch one another. The cut 
ends of one of the heavy insulated supply wires are then soldered 
to the two ends of the coil. The coil with the soldered junctions is 
then immersed in a glass or porcelain dish containing pure water, 
no salt being used (fig. 283). If the current is to be on for some 
time it is a great advantage to have the vessel containing the rheo- 
stat stand in a sink or in some place where water can drain away, 
and then to keep a stream of cold water flowing into the vessel to 
keep the wire cool. 

This general scheme is used in making tests of the gigantic 
generators used in large power plants. For such tests the wire used 
is naked telegraph wire of the right resistance and length laid out 



371a. With such a bucket rheostat, 12 liters (12 quarts) of l / 2 % salt 
solution were used, and the distance between the iron discs could be as great 
as 15 cm. (6 in.). With the discs 15 cm. apart and the solution at 23 centi- 
grade a current of 10 amperes flowed. After an hour, when the temperature 
had risen to 43 C., 12 amperes of current flowed. With the discs nearly in 
contact 20 amperes were given. 

In this experiment the iron discs were 18 cm. (7 in.) in diameter. By in- 
creasing the size of the iron discs the current could be increased, and by 
diminishing it the current could be diminished. Iron (tin) funnels are some- 
times used instead of discs. It is safer to have both discs covered with the 
burlaps, and the conducting wires soldered to the discs or funnels. 



CH. XIII] 
W, 



RHEOSTATS AND OTHER BALLAST 



529 



Wi 




FIG. 284. ADJ USTABLE RHEOSTAT 
MADE OF SHEETS OF TIN. 

A, B, C, D Clip-connectors to hold 
the ends of the wires. 

Permanent connectors c n 1-2 are 
used to join the further ends of the tin 
strips 1-2 and 3-4 and a connector (c n) 
is used between B and C. 

J, J' Movable adjusters to include more or less of the resistance in the 
circuit and thus increase or diminish the amperage. 

This rheostat is composed of four sheets of tin cut as shown in fig. 285. It is, 
therefore, four rheostats in series (see fig. 287). As here connected all four 
sheets are used. By putting supply wire W 2 from A to C or from D to B only 
two of the sheets would be used. Then by means of the adjusters / and /' the 
amount of resistance can be increased or diminished at will. 

The small diagram at the left shows how the pairs of strips of each side are 
connected with each other at the far end. 

At the near end of the frame the arched wire connects the two pairs of plates 
of both sides at B and C. 



530 



RHEOSTATS AND OTHER BALLAST 



[Cn. XIII 









FIG. 285. To SHOW THE TIN PLATE CUT 
IN INCOMPLETE STRIPS FOR THE 
RHEOSTAT. 

Cut in this way the tin plate is like a 
continuous flat wire. 



straight in the bottom of a 
river or creek. The flowing 
water keeps the resistance 
wire cool. 

733. Home-made rheo- 
stat of tin strips. A good 
adjustable rheostat for experi- 
mental purposes can be cheap- 
ly made by cutting tinned 
sheet iron into strips as shown 
in figure 284, 285, and nail- 
ing these strips to a wooden 
frame. One end of the con- 
ductor is fastened to one end 
of the sheet, and the other to 
the other end of the sheet. 

To make this an adjustable 
of heavy copper wire or of sheet copper 
to the other as shown. By this 



rheostat, a "juniper 
is put across from one sheet 
means the current can be sent through as much or as little of 
the resistance as desired, thus giving a great range in the 
amperage. As the surface is very great in the thin sheet iron, the 
air currents carry off the heat developed so that this rheostat does 
not become unduly heated. It is a very common form around 
physical laboratories, but is bulky and not very well adapted to a 
magic lantern or a moving picture installation. Furthermore, such 
a rheostat does not fulfill the requirements of the National Elec- 
trical Code, as there is too much 
combustible material in connec- 
tion with it, and the resistance 
is not boxed in. 

734. Rheostats in series. 
If one has two rheostats, less 
current will be allowed to flow FIG. 286. 
if they are connected to the line 
in series, that is, so that all the 




AN ELECTRIC CIRCUIT AND 
GENERATOR. 



C Generator. 
A Arc Lamp, 
current must flow through both R Rheostat. 



CH. XIII] 



RHEOSTATS AND OTHER BALLAST 



531 




rheostats. According to Ohm's 
law ( 724a), the amount of cur- 
rent varies inversely as the resis- 
tance, then if two equal rheo- 
stats were used only half as 
much current would flow as FlG 28? RHEO STATS IN SERIES. 
when one rheostat is used. Also c Dynamo, 
if the voltage is increased the A Arc lamp. 

i-, ,-, R,, R, Rheostats in series, all the 

amperage will increase in the cun4nt must pass through both of them 
same ratio provided the resis- (compare fig. 288). 

mi The two rheostats R, and R, are con- 

tance remains constant. Then nec ted in series to get a smaller current 
if one has two rheostats, each than can be obtained by the use of one 
of the right capacity for an arc 

lamp with a 1 10 volt circuit, the two in series would give approxi- 
mately the correct number of amperes on a 220 volt circuit. The 
amperage would be somewhat higher on the 220 volt circuit because 
when used singly on a no volt circuit each is somewhat reinforced 
by the resistance of the arc lamp. When both are used for one 
lamp on a 220 volt circuit there is not twice the resistance, hence 
the amperage will be somewhat greater than with one rhostat on 
a no volt circuit. 



735. Rheostats in parallel. 

parallel as shown in fig. 288, two 




FIG. 288. Two RHEOSTATS IN PAR- 
ALLEL, GIVING Two PATHS 
FOR THE CURRENT. 

G Dynamo. 

A Arc lamp. 

R It R 2 Rheostats in parallel. 

With two or more paths for the cur- 
rent, the total amperage will be the 
sum of the amperages going over each 
path ( 735). 



If two rheostats are inserted in 
paths are furnished for the cur- 
rent. The amperage given by 
both will be the sum of that given 
by each separately, for example, 
if one had two fixed rheostats, 
each one giving five amperes of 
current, if they were connected 
with the line in parallel, 10 am- 
peres would be allowed to flow. 
On the other hand if they were 
connected in series (fig. 287) so 
that all the current had to flow 
through both of them then only 
2^2 amperes of current would be 
available. (See 724 a). 



532 RHEOSTATS AND OTHER BALLAST [Cn. XIII 

736. Reactors, inductors, choke-coils, economy-coils, com- 
pensator-coils, etc. When alternating current is used the wasteful 
method of current control by means of a resistor or rheostat where 
so much electrical energy is transformed into heat should be 
avoided whenever possible. 

In place of a rheostat such as is described above ( 723 + ) an 
inductor is used. This consists of a soft -iron core around which is 
wound a coil of insulated wire. The alternating current passes 
through this coil; this alternately magnetizes and demagnetizes 
the soft-iron core and limits the flow of the current. But the 
energy is not dissipated, for the energy used in magnetizing the 
core is given up again when the core is demagnetized. It is true 
that a small amount of the energy is wasted in heating the appar- 
atus, but the amount is so small (5% to 8%) as compared with that 
lost in a rheostat that it is negligible. 

Variable amperage can be obtained with an inductor by having 
the soft-iron core movable so that a greater or less amount of it 
will be within the coil. 

The more of the soft -iron core within the coil the greater will be 
the inductance and hence the less the amperage; and conversely, 
the less of the soft-iron core within the coil the less will be the 
inductance and the greater the amperage. In fig. 197 the core 
is only partly inserted in the coil and a medium amount of current 
is therefore allowed to flow. 




1 



737. Wiring the inductor and transformer. The inductor is 

inserted along one wire (in series) 
exactly as the rheostat is inserted 
(fig. 289). With a special arc 
lamp transformer the line is con- 
nected to the primary of the trans- 
former and the arc lamp is con- 
FIG. 289. INDUCTOR IN SERIES WITH j , ,, , .^, 

AN ARC LAMP. nected to the secondary without 

G Dynamo. the use of resistance (fig. 290). 

=t Alternating current circuit. 

A Arc lamp with right-angle car- 738. Comparison of the 

bons - T ... amount of energy used with an 

L Inductor to serve as ballast with 

alternating current. inductor and with a rheostat. (A) 



CH. XIII] RHEOSTATS AND OTHER BALLAST 533 

With an inductor. Let the line voltage be no and the amper- 
age 55 as shown by the ammeter; the voltage across the arc will 
be 34 volts. The power consumption will be volts times amperes, 
that is, in this case, 34 x 55 = 1870 watts or 1.87 kilowatts. As 
the inductor does not absorb an appreciable amount of energy, 
the 1.87 kilowatts represents the energy needed to produce the 
arc light. 

(B) With a rheostat. If now a rheostat is used, the watt- 
meter will record not only the energy required to maintain the arc 
light, but also the energy wasted in heating the rheostat. 

For example, suppose as above that the line voltage is no, the 
amperage 55, and the voltage across the arc is 34. Then as before 
the arc light requires 34 x 55 = 1870 watts or 1.87 kilowatts. 

But the difference between the 34 volts at the arc and the no 
volts in the line (76 volts) is used in heating the rheostat. 

The energy used in heating the rheostat is then 76x55 = 4180 
watts or 4.18 kilowatts. Both this wasted energy as well as the 
actual energy used in the arc will be recorded on the wattmeter 
and the user of the arc lamp will have to pay for 1.87 + 4.18 or 6. 05 
kilowatts to run his lamp instead of the 1.87 kilowatts when the 
inductor is used. That is it will cost more than three times as 
much to run the arc lamp with a rheostat as with an inductor or 
choke-coil. 

STATIONARY TRANSFORMER FOR ALTERNATING CURRENT 

739. Transformer. A transformer is a device for changing 
the voltage of an alternating electric current. This change may 
be an increase in the voltage step-up transformer, or a decrease 
in the voltage step-down transformer. The device consists in a 
soft -iron ring wound with coils of insulated wire. In the simplest 



738a There is no simple method of economizing with direct current 
comparable with the use of an inductor with alternating current. Sometimes 
when one must draw on a current at 220 volts pressure there is used a motor 
generator set. The motor is driven by the 220 volts current and the genera- 
tor produces current at 60 to 70 volts pressure. At this voltage only a 
limited amount of resistance is necessary ( 747), and there is some saving, 
but not so much as by using an inductor with alternating current. 



534 RHEOSTATS AND OTHER BALLAST [H. XIII 

case there are two coils (fig. 291). If an alternating current supply 
is connected with the primary coil an alternating current can be 
drawn from the secondary coil. 

The voltage and amperage 
which can be drawn from the 
secondary coil will depend upon 
the electric supply and upon the 
relative number of turns of wire 
FIG. 290. USE OF A SPECIAL TRANS- in the primary and in the second- 
FORMER WITH AN ARC LAMP. ary co ii s . jf t he number of turns 
G Dynamo. is the same in both, then the 

? anSe^ UrrentCirCUit ' voltage and amperage remain 
A Arc lamp. practically the same as if the 

The primary of the transformer is -i T , , 

connected to the dynamo while the colls were not Present. In other 
secondary is connected to the arc words the circuit is in every way 

la The transformer has sufficient "re- almost as if the wire were contin - 
actance" to serve as a ballast for the uous. If the transformer were 




tO "* " * SteP " Wn Perfect the voltage and amperage 
would be exactly the same as if it 

were not present. In practice they are a little less, but a good 
transformer gives an efficiency of 95% to 98%. 

If the secondary coil has a different number of turns from the 
primary coil then the voltage will vary directly as the ratio of the 
number of turns in the two coils, and the amperage will vary 
inversely as that ratio. That is, assuming that there is no loss in 
the transformer, the watts delivered will remain constant as the 
product of volts x amperes remains the same. 

For example, suppose the secondary coil has %'th as many turns 
as the primary coil, then the number of volts across the secondary 
will be %ih the number across the primary and the number of 
amperes delivered by the secondary will be four times the number 
drawn by the primary. If now the primary is connected to a 220 
volt line there will be a potential difference of one-fourth that 
number or 55 volts across the terminals of the secondary coil. 
Suppose the secondary coil supplies 60 amperes, as might be the 
case with an arc lamp, then the primary coil would draw one-fourth 



CH. XIII] THE ELECTRIC ARC 535 

of this number, or 15 amperes from the line. The watts in the two 
cases are theoretically exactly the same. 

The watts for the primary are 220x15 = 33 - 

The watts for the secondary are 55 x 60 = 3300. 

(1) Volts secondary _ Turns secondary 

Volts primary Turns primary 

(2) Amperes primary Turns secondary 
Amperes secondary Turns primary 




FIG. 291. DIAGRAM OF A TRANSFORMER. 

Two coils of a wire, Primary and Secondary, are wound on an iron ring. An 
alternating current in the primary sets up an alternating magnetic flux in the 
iron ring, which in turn sets up an alternating electric potential in the secondary 
coil. 

THE ELECTRIC ARC 

740. The construction of an electric arc is very simple. Two 
electrodes are taken which may be made of any conducting material. 
One electrode is connected directly to one of the wires of a direct 
current supply of over 40 volts, the other electrode is connected 
through a rheostat to the other wire (fig. 280). When the two 
electrodes are brought in contact an electric current will flow 
between them. If now, the electrodes are slightly separated, the 
current will not be immediately interrupted, but will flow through 
the air gap between the electrodes. 



536 THE ELECTRIC ARC [Cn. XIII 

The exact nature of the resulting phenomenon will depend upon 
the material of which the electrodes are made, upon the voltage of 
the current supply and the resistance of the rheostat, and the kind 
of gas surrounding the electrodes. 

741. Arc lamp. Any arrangement for holding the electrodes 
and feeding them together as they wear away may be called an arc 
lamp. 

It consists of three essential elements: (i) A clamp for holding 
the positive electrode; (2) A clamp for holding the negative elec- 
trode; (3) A mechanism for moving the holders and therefore the 
electrodes nearer together or separating them farther apart. 

The electrode holders must be insulated so that the current must 
flow through the electrodes and not follow any short circuits (fig. 
270). 

For the hand-feed and the automatic types of arc lamps see 
Chapter I, 9-11. 

742. With direct current, the arc is made up of three parts. 

1. The arc stream; a highly heated, incandescent gas which 
conducts the current between the electrodes. 

2. The positive crater; where the current leaves the positive 
electrode to enter the arc stream. 

3 . The negative crater ; where the current leaves the arc stream 
to enter the negative electrode (fig. 292). 

743. Electrical behavior of the direct current arc. Measure- 
ment of the voltage drop in various parts of the carbon arc reveals 
the fact that the potential difference between the two electrodes 
( 743a) is made up of three parts. Starting from the positive 
side, the potential difference between the positive electrode and the 
arc stream is about 32 volts. The potential difference between the 
arc stream and the negative electrode is about 9 volts, thus the 
potential difference between the electrodes with the shortest possi- 
ble arc is about 41 volts ( 743b). 

As the arc is lengthened there is an additional drop in potential 
in the arc stream which depends mainly on the length, but partly on 
the cross section of the arc stream. As the arc length is changed, 



CH. XIII] 



THE ELECTRIC ARC 



537 




FIG. 292. 



THE VERTICAL CARBON ARC WITH 20 AMPERES OF DIRECT 
CURRENT. 



a Vertical carbons with the positive carbon above and the negative carbon 
below. This shows that the large crater is on the positive carbon and the small 
crater on the negative carbon. Between the two craters extends the arc stream 
of hot gases. 

This photograph was made with an exposure of i/ioo second, the aperture 
being F/22. A color screen was used to cut out most of the violet, so that the 
arc stream would not obscure the craters. A subsequent exposure of 90 seconds 
was made without a color screen and with an aperture of F/8. The illumina- 
tion during this exposure was by means of a 40 watt, mazda lamp. 

b Vertical carbons with a 20 ampere direct current. No color screen. 
Exposure i/ioo sec.; opening F/22. 

This shows the size of the two craters ; it also shows the conical arc stream 
almost as light as the craters. This is because the violet light which has 
relativelv little effect in illumination has a great effect on the photographic 
plate. 

This picture shows how the carbons, the craters and the arc stream appear 
in an instantaneous view to the photographic plate, while the one at the left 
(a) gives much more nearly the appearance to the human eye. with an instan- 
taneous view. 



538 



THE ELECTRIC ARC 



[Ca. XIII 



the change in voltage is almost entirely due to the change in the 
length of the arc stream. 

When the arc is of medium length, as for use in projection, the 
potential difference between the two carbons averages about 55 
volts. This would mean that there is a drop of 32 volts between 
the positive carbon and the upper end of the arc stream, a drop of 
14 volts between the upper and lower ends of the arc stream, and 9 
volts between the lower end of the arc stream and the negative 
carbon. 

If the electrodes are made of other substances than carbon, the 
potential drop is differently distributed. Thus in the "Luminous" 




FIG. 293. 



SIDE VIEW OF THE RIGHT-ANGLE CARBON ARC WITH 10 and WITH 
20 AMPERES OF DIRECT CURRENT 



A 10 ampere arc, B 20 ampere arc. The size of the positive crater is 
markedly larger with the higher amperage. 

The lower pictures were made by an instantaneous exposure. 

The upper pictures were made by a double exposure, that is, an instantaneous 
exposure with the current on, to show the craters and the arc stream, and then 
an additional exposure of 90 seconds with the current off to bring out the car- 
bons. For the second exposure a 40 watt, mazda lamp was used for illuminat- 
ing the carbons. 



CH. XIII] THE ELECTRIC ARC 539 

arc which consists of a copper positive electrode and a negative 
electrode made of a mixture of iron and titanium oxides, the lowest 
arc voltage is about 30 volts. The lowest arc potential between 
electrodes of other substances than carbon are, magnetite 30; 
platinum 27; iron 26; nickel 26; copper 23; silver 15; zinc 16; 
cadmium 16; mercury 13. 

The potential differences in the arc lamp are practically constant 
no matter what current is flowing, but there is a small change with 
change in current. This is generally such that the greater the 
current the less the potential difference, and may be explained as 
follows : 

Suppose a current of 10 amperes to be flowing between the two 
electrodes of an arc lamp. This will be carried by a small cone 
shaped mass of conducting gas (fig. 293 A). If the current is 
increased to 20 amperes the extra heat developed is sufficient to 
bring more air to a high enough temperature to conduct current, 
and the cone of conducting gas increases in diameter (fig. 293 B). 

A large cone of conducting gas will be losing heat at a relatively 
less rate than will a small cone, hence its temperature will be higher 
and its resistance will be less. As a result of the increased con- 
ductivity of the hot gases of the arc stream, the greater the current 
the lower will be the potential difference between the electrodes. 
There is also a slight lowering of the contact potential difference 
between the electrodes and the arc stream as well as a lessening of 
the potential drop in the arc stream. 

THE USE OF BALLAST 

744. The need of a ballast in series with the arc to control the 
current. On account of the peculiar electrical behavior of the arc 
lamp it is necessary to use a ballast such as rheostat, or an inductor 
in series with the arc, or else to use an especially designed generator. 

743a. While the two electrodes of an arc lamp may be of any conducting 
material, with projection arc lamps the electrodes are always made of carbon 
and are generally referred to simply as carbons. 

743b. These figures are approximations and vary slightly with arc 
length and current but are general averages for the usual arc lengths employed : 
3 to 10 mm. 

See Mrs. Ayrton, The Electric Arc. 



540 USE OF BALLAST WITH ARC LAMPS [Cn. XIII 

With a metallic wire, the resistance is nearly constant, and the 
potential difference is greater the greater the current flowing. Any 
change in resistance is due to the rise of temperature when a current 
is flowing. The higher the temperature, the greater the resistance. 
An arc, on the other hand, has no definite resistance, but its resist- 
ance varies with the current flowing. This variation is such that 




A B 

FIG. 294. FACE AND LATERAL VIEWS OF THE RIGHT-ANGLE CARBON ARC 
WITH 10 AND WITH 2O AMPERES OF DIRECT CURRENT. 

A With 10 amperes, B with 20 amperes of direct current. 

The size of the crater in the two cases is very strikingly brought out. 

The middle figures had an additional exposure to bring out the carbons (see 
fig. 292-293), while the lateral views above and the front views below had only 
an instantaneous exposure. 

The positive crater above and the negative crater below are clearly brought 
out in all the pictures (see fig. 292). 



CH. XIII] USE OF BALLAST WITH ARC LAMPS 541 

the potential difference across the arc remains nearly the same 
regardless of how much current is flowing. 

The commercial electric supply is designed to furnish current for 
incandescent lamps, and is maintained at a nearly constant voltage 
no matter how much current is used. The arc lamp, on the other 
hand, is to be supplied by a constant current. If one were to 
attempt to connect an arc directly to the terminals of the supply 




FIG. 295. LATERAL AND FACE VIEW OF THE RIGHT-ANGLE CARBON ARC WITH 
20 AMPERES OF DIRECT CURRENT. 

No color screen was used with the lateral view so that the arc stream would 
show. In the front view a color screen was used to bring out clearly the large 
positive crater above and the small negative crater below. 

This figure is for comparison with the alternating current arc in fig. 296. 

To bring out the carbons, an additional exposure was made as for fig. 292- 
293- 

line without an intermediate rheostat, as soon as the two electrodes 
were brought in contact an extremely large current would flow. 
Theoretically, this current would be infinite, but practically the 
flow is limited by the very small resistance of the supply wires and 
the capacity of the dynamo. In a modern installation the current 
would be immediately interrupted by the circuit breakers and burn- 



542 



USE OF BALLAST WITH ARC LAMPS 



[CH. XIII 



ing out of the fuses before any serious damage could result. Even 
after the arc is burning, if one were to remove the resistance by 
short-circuiting it, the current would increase to an enormous 
value. 

745. Example with 110 volt supply, using a rheostat. If we 
assume that the arc is of such a length that the potential difference 
between the electrodes is 10 volts, and that this potential difference 




FIG. 296. LATERAL AND FACE VIEWS OF THE RIGHT-ANGLE CARBON ARC 
WITH 25 AMPERES OF ALTERNATING CURRENT. 

By comparing this picture with fig. 295 it will be seen that in this both 
craters are of the same size; and that, although 25 amperes of current are 
flowing, the crater on the upper carbon from which the light is derived is much 
smaller than with the direct current. The sizes of the upper crater give a good 
idea of the amount of illumination furnished in the two cases. 

An additional exposure was made to bring out the carbons as in fig. 292-293. 

remains practically the same if the current is diminished or in- 
creased, and if the supply is no volts, and that this voltage is 
practically independent of the current used, it is evident that 
between one of the electrodes and one of the supply wires there must 
be a potential drop of 60 volts. By using a rheostat at this point 
the current is controlled. Thus suppose that the rheostat has a 
resistance of 6 ohms, then according to Ohm's law ( 724a), as the 
potential difference across its terminals is 60 volts, the current will 



CH. XIII] 



USE OF BALLAST WITH ARC LAMPS 



543 



be 10 amperes, V/O = A. Now suppose the arc length were 
changed say by bringing the electrodes in contact. In this case 
there would be the full line voltage, no volts across the rheostat 
and the current would be 1 10/6 = 18.3 amperes. Suppose the arc 
length were increased until the potential at the arc was 60 volts. 
The potential across the rheostat would then be no 60 = 50 
volts. The current would then be 50/6 = 8.2 amperes. In this 
example the conditions are what is known as stable, that is, as the 
arc length is decreased the current is increased, but does not reach 
an infinite value, and as the arc length is increased the current 
decreases but it does not become zero. 




FIG. 297. 



LATERAL AND FACE VIEWS OF AN INCLINED CARBON ARC WITH 20 
AMPERES OF DIRECT CURRENT. 



This picture shows that with the inclined carbons in proper position, the 
positive crater on the upper carbon faces toward the condenser. It is evident 
also that as the carbon burns away the crater will get farther and farther above 
the principal axis of the projection apparatus. 

An additional exposure was made to bring out the carbons as with fi'g. 292- 
293- 



544 USE OF BALLAST WITH ARC LAMPS [Cn. XIII 

746. Line voltage exactly equal to arc voltage. It would 
appear that it might be desirable to use a line voltage of exactly 
what is required by the arc and omit the rheostat. Suppose in the 
above example that this were done by using a line voltage of 50 
volts. Now as the arc voltage is constantly varying owing to slight 
irregularities in the carbons, to the wearing away of the carbons and 
to other causes, it is evident that for an instant the arc voltage 
might drop below 50 volts or it might rise above 50 volts. If the 
arc voltage should rise above 50 volts, the arc would immediately go 
out as the supply is but 50 volts, and if the arc voltage should drop 
slightly below this value, the current would rapidly increase. The 
result would be that the arc would either go out or else would act 
like a short circuit. In this example the conditions are unstable; 
that is, no definite current can be maintained. 

747. Intermediate voltage. In practice an intermediate 
voltage is sometimes used, that is, dynamos to be used for projector 
arcs are sometimes designed for about 70 volts. Here the arc is 
sufficiently stable for practical purposes but requires more atten- 
tion than with the higher supply voltage. Taking the above 
example. The arc voltage at 50 volts leaves 20 volts across the 
rheostat. To give 10 amperes requires 20/10 = 2 ohms resistance. 
If now the electrodes are brought in contact to start the arc the 
current will be limited only by the resistance in the rheostat and 
the current will be 70/2 =35 amperes. If the arc gets long enough 
to take 60 volts, the difference to be taken up in the rheostat is but 
10 volts, and the current will drop off to 10/2 = 5 amperes. This, 
therefore, means that with the smaller margin between the line 
voltage and the arc voltage, the arc becomes less stable. 

748. Ballast with alternating current. With alternating 
current, an inductor (choke-coil) is often used instead of a rheostat. 
This behaves as a ballast in a somewhat similar way to the rheostat 
but to explain the exact process of regulation would require a more 
exhaustive discussion of alternating currents than is justified in 
this book, but see 736. 



CH. XIII] 



USE OF BALLAST WITH ARC LAMPS 



545 




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THE INCLI 
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This show 
20 ampere: 
-he carbons 


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546 LIGHT FROM THE ARC [Cn. XIII 

THE LIGHT PRODUCTION OF THE ARC 

749. Cause of light from the arc. The light production from 
the carbon arc is due entirely to the high temperature to which the 
tips of the carbons are raised, i. e., they become white hot. The 
practical problem in projection with the arc deals with the best 
method of producing this white heat and of utilizing it. 

When the electric current passes between the two electrodes the 
heating effect in the different parts is proportional to the power 
consumed in them. 

The current being the same in all parts, the heating effect must 
be in proportion to the potential drop (or voltage consumed) in the 
different parts. 

Counting the total drop 55 volts, it is divided into: 

+ crater drop = 32 volts = 58% 

crater drop = 9 volts = 17% 

arc stream = 14 volts = 25% 

Total, 55 volts 100% 

We see from this that the heating effect will occur principally 
at the positive carbon. 

Carbon being rather a poor conductor of heat, the heat generated 
within the small area of the crater must escape mainly by radia- 
tion. 

At the negative electrode the heat production is less rapid and 
not so high a temperature is reached. 

Between the electrodes the heat production is fairly rapid, but 
the hot gases of the arc stream with the carbon arc are nearly 
transparent and radiate energy very slowly. 

Furthermore the violet lines of the spectrum in the arc stream 
are brighter than from the crater itself ( 749 a). 

750. Temperature of the crater. The temperature of the 
positive crater rises until such a temperature is reached that carbon 

749a. The great brilliancy of the violet lines in the arc stream has received 
two explanations: (i) That the arc stream is higher in temperature than even 
the crater itself; (2) That the electric current passing through the gas causes 
the gas to glow irrespective of its temperature. That is, it causes electro- 
luminescence as in the vacuum tube or the aurora borealis. 



CH. XIII] LIGHT FROM THE ARC 547 

is volatilized. This is the highest temperature which it is possible 
to obtain artificially. The temperature of the positive crater of the 
carbon arc has been estimated at about 3700 absolute, that is, 
3427 Centigrade or 6200 Fahrenheit ( 7Soa). Compare this 
with the temperature of the sun, about 6750 absolute, 6477 C; 
the acetylene flame, 2330 absolute, 2057 C.; the gas flame, 1830 
absolute, 1557 C. ( 75ob). 

751. Parts of the light source. Considered as a light source, 
the direct current arc may be divided into four parts. 

1. The positive crater. 

2 . The negative crater. 

3. The hot ends of the carbons adjacent to the craters. 

4. The arc stream. 

The light emitted by the hot electrodes depends upon their vis- 
ible radiation being approximately proportional to the 5th power 
of their absolute temperature. The positive crater is the hottest 
part of the arc and furnishes most of the light. The negative crater 
furnishes much less light than the positive crater, being smaller and 
not as hot. 

The carbons are white hot for some distance away from the 
craters and furnish some of the light of the arc. In calculating the 
total light from the arc it would be necessary to consider the entire 
area included between the line surrounding the positive carbon 
which is at red heat and the corresponding line on the negative 
carbon. 

The arc stream with the carbon arc emits but little useful light. 
When flame-arc carbons are used, however, the greater part of the 

750a. Bulletin of the Bureau of Standards, Vol. i, p. 909 and reprint 8. 

750b. Absolute temperature. The absolute zero is defined as the tem- 
perature at which a perfect gas would exert no pressure. This is about -2 73 
centigrade, i. e., 273 centigrade below the melting point of ice. In calcula- 
tions of high temperature and radiation, all formulae are based on absolute 
temperature, that is, the temperatures where the zero is the absolute zero and 
where the degree is the degree centigrade. 

To find the absolute temperature of a body add 273 to its temperature on 
the centigrade scale. Thus ice melts at o centigrade or 273 absolute, and 
water boils at 100 centigrade or 373 absolute. The temperature of the 
human body, 37.5 C. is 310.5 absolute. If the absolute temperature is given, 
subtract 273 from this value to find the centigrade reading. 



548 



LIGHT FROM THE ARC 



[Cn. XIII 




FIG. 299 



CH. XIII] LIGHT FROM THE ARC 549 

FIG. 299. SIDE AND FRONT VIEWS OF THE INCLINED CARBON ARC WITH 15 
AMPERES OF DIRECT CURRENT (EWON'S AUTOMATIC LAMP). 

The upper carbon (+c) is soft-cored and 18 mm. in diameter; the lower 
carbon ( c) is solid and 12 mm. in diameter. 

This is to illustrate an automatic lamp with a magnet (m) to control the 
magnetic blow; the use of a large, cored upper carbon (+c) 18 mm. in diame- 
ter; and a small solid lower or negative carbon ( c) 12 mm. in diameter. 

Incidentally there is shown the wandering of the crater in the right hand 
lower picture. When the crater wanders in this way the source of light is 
outside the principal optic axis. 

Photographed with an instantaneous exposure for the arcs and with an 
additional exposure of 90 seconds for the carbons and the blow magnet (see fig. 
292-293). 

light is furnished by the incandescent gases of the arc stream. 
Flame-arc carbons are not ordinarily used in projection. 

For purposes of projection, only the light from the positive crater 
of the direct current arc, or usually from only one of the craters of 
an alternating current arc need be considered. The large objective 
of the rragic lantern utilizes the light from both carbons with 
alternating current and this is important. 

752. The alternating current arc. Most conducting materials 
when used as the terminals of an arc lamp will not allow a reversal 
or even a very short interruption of the current without going out. 
This property is used in the mercury arc rectifier. 

When carbon electrodes are used, however, the current may be 
interrupted for a short interval, or the current may be reversed 
without putting out the arc. 

When the alternating current is used, first one carbon and then 
the other is positive. Craters of equal intensity are formed on both 
carbons, but neither is as bright nor as large as is the single positive 
crater when direct current of the same amperage is used. 

The light from a single crater is not steady but is intermittent. 

The process during one cycle can be described as follows : 

When the current is reversed so that, say, the upper carbon 
becomes positive, the crater is fairly cool. For the short time it is 
the positive crater, its temperature rises very rapidly. Whether 
or not it momentarily reaches the temperature which it would if 
permanently the positive crater is uncertain. The current dies 
out and the crater cools rapidly. When the current has reversed 



550 LIGHT FROM THE ARC CH. XIII 

its direction the crater is negative. The heating effect of the 
current is small and the carbon tip continues to cool until the 
current has again died out. This cooling still continues until the 
current has again reversed its direction, and increased to a con- 
siderable positive value. 

The temperature of an alternating current arc crater is at no 
instant higher than that of a direct current arc crater with the same 
amperage, and, as part of the time its temperature is much lower 
than this, the average temperature will be lower than with a 
direct current crater, hence the light will be less and of a yellower 
color. 








I 



FIG. 300. SOME POSITIONS OF THE CARBON ELECTRODES USED IN PROJECTION 

LAMPS. 

A Vertical carbons. This position gives the least light along the principal 
optic axis. 

B Inclined carbons. 

C Horizontal carbons. This arrangement is common for the search light, 
and for the reflectors used in projection (see fig. 95). 

D The usual arrangement for the carbons when at right angles. The 
upper or horizontal carbon is positive with direct current. '1 he crater on it is 
in the optic axis and serves as the source of light with both direct and alter- 
nating current. 

E Right-angle carbons in which the horizontal, positive carbon is below. 
This is an unusual arrangement. 

V V-arrangement of the carbons for alternating current. With this 
arrangement both craters supply light for the projection of lantern slides or 
opaque objects. 

THE ARC LAMP AS AN ILLUMINANT 

753. The arc lamps suitable for projection purposes may have 
the carbons in any one of five positions. 

1. With inclined carbons (fig. 297). 

2. With carbons at right angles (fig. 295). 

3. With converging carbons (fig. 300). 

4. With vertical carbons (fig. 292). 

5. With horizontal carbons along the axis (fig. 300). 



CH. XIII] 



LIGHT FROM THE ARC 



551 



Most of the light from the arc, and all of the light which is useful 
for projection comes from the craters of the arc ; from the positive 
crater, if a direct current arc. 

753a. Table showing the proper size of cored-carbons for 
different amperages, and the rate of wear in millimeters per 
hour; also the relative rate of burning in length and in weight. 

(For the small carbons to be used on the house electric light- 
ing system see 123, 131, 417-418.) 

Direct Current, Right-Angled Carbons. 

AUTOMATIC LAMP 



Amperes 


Size Carbons 


Burns, mm. 
per Hour 


Relative 
Volume 


Relative 
Burning Weight 


IO 


II upper 


37 


1.8 






8 lower 


39 


i 




15 


ii " 


47 


1.65 


1.69 




8 


54 


i 


I 


15 


14 " 


27 


i-75 


1.6 7 




ii 


25 


i 


I 


2O 


14 " 


36 


1.87 


1.77 




ii 


3i 


i 


I 


20 


14 " 


36 


i-53 


1.36 




ii 


38 


i 


I 


25 


14 " 


41 


i-95 


1.92 




ii 


34 


i 


I 


25 


14 " 


40 


1.61 


I-3I 




ii 


40 


i 


I 



HAND-FEED LAMP 



15 


II 


53 


1.6 5 


I-5I 


20 


II 
14 " 


32 
37 


I 
1.62 


I 


40 


H 
I 4 " 


23 
48 


I 
2.00 


2.OO 


40 


H 

15 " 


24 
44 


I 
2.2 


I 
2.26 




15 


20 


I 


I 



552 



LIGHT FROM THE ARC 
Direct Current, Inclined Carbons. 



[Cn. XIII 



HAND-FEED LAMP. 


Amperes 


Size Carbons 


Bums mm. 
per Hour 


Relative 
Volume 


Relative 
Burning Weight 


15 


14 upper 
ii lower 


41 
41 


1.62 
I 


_L33 
i 


2O 


14 " 


37 


1.84 


i-77 




ii 


34 


I 


i 


25 


14 " 




39 


I. 




1 .81 


ii 


33 




i 


40 


i 8 cored 
12 solid 


26 
32 


1.8 


2.32 


i 


i 


15 


14 " 


28 


'4 s 


1.72 




*4 


19 


I 


i 


40 


14 " 


54 


2-4 


2.32 




H 


22 


I 


i 


Alternating Current, Hand-Feed Lamp. 


RIGHT-ANGLED CARBONS. 


Ampeies 


Size Carbons 


Burns, mm. per Hour 


20 


1 1 upper 
ii lower 


.34 

34 


20 


14 " 


20 




14 


20 


25 


14 " 


20 




H 


2O 


INCLINED CARBONS 


25 


14 upper 
14 lower 


26 


30 


15 " 
15 " 


"30 


35 


15 " 


24 




15 


30 


40 


16 " 


~28 



CH. XIII] CANDLE-POWER OF ARC LAMPS 



CANDLE-POWER OF ARC LAMPS 



553 



754. A number of measurements of the candle-power of arc 
lamps have been made, partly in the Physical Laboratory at 
Cornell University, and partly in the Illuminating Engineering 
Laboratory of the General Electric Company at Schenectady. 
The experiments made at Cornell were for the higher currents and 
were made primarily to ascertain the efficiency of the mercury arc 
rectifier and the power consumption with different forms of ballast 
( 754a). 




FIG. 301. 



CARBONS IN THE CORRECT RELATIVE POSITION FOR BOTH DIRECT 
AND ALTERNATING CURRENTS. 



A Inclined carbons in the correct position for alternating current. 

B Inclined carbons in the correct position for direct current. 

C Carbons at right angles in the correct position for either direct or 
alternating current. Direct current is indicated. 

D Carbons arranged in a V-shaped position. For this position alternating 
current only is employed; and the crater on each carbon contributes to the 
light. The V may be either in a vertical or in a horizontal plane. The ver- 
tical arrangement is the more common. 



755. Variation of Candle-Power with current. Candle-power 
measurements were made in the horizontal direction, that is, along 
the axis of the lantern, using different currents and with both the 
right-angle and the inclined-carbon arrangements. Great care 
was taken to hold the position of the electrodes and craters as 
shown in fig. 301, as these positions furnish the greatest amount of 
light. With direct current especially, it is necessary that the crater 

754a. The results of the Schenectady tests were published in the Electrical 
World, Oct. 13, 1911. 



554 



CANDLE-POWER OF ARC LAMPS 



[Cn. XIII 



m 



/< 



- 







V? 












X 



IS 



FIG. 302. VARIATION IN INTENSITY OF LIGHT FROM PROJECTION ARC LAMPS 
WITH DIRECT AND WITH ALTERNATING CURRENT. 

x Right-angle arc. 

o Inclined carbon arc. 

The small dotted curve is for small currents with the right-angle arc burn- 
ing 6mm. carbons. 

As shown by these curves, the right-angle arc lamp gives a greater candle- 
power for the same current than does the inclined carbon arc lamp, with both 
direct and alternating current. Also that direct current gives about four times 
the light that the same number of amperes of alternating current gives. 



CH. XIII] CANDLE-POWER OF ARC LAMPS 555 

face forward as is shown in figure 297 for the inclined electrodes and 
in figures 294-296 for the right-angle arc. The results of these 
measurements are shown in curve form in fig. 302. These curves 
show that the greater the current the greater is the amount of light 
given by the arc. The increase in light is, however, more rapid 
than the increase in current and no simple mathematical statement 
of the relationship is possible. The crosses indicate the individual 
measurements with the right-angle arrangement and the circles, 
measurements with the inclined carbon arc. The upper curve is for 
the right-angle arc with direct current. In this case the highest 
candle-power for the same current (amperage) is obtained. The 
next curve is for the inclined carbon arc with direct current. The 
light is not quite as much with this arrangement as with the right- 
angle arc. 

The two lower curves are for alternating current. It will be 
noticed that there is a greater difference in candle-power depending 
on the electrode arrangement with alternating than with direct 
current. The short dotted part of the curve for the right-angle 
arrangement is for 6 mm. carbons and small currents, while the 
main part of the curve is for larger carbons. 

A table showing the results of the individual measurements 
might be misleading, as large variations in the light of the arc are 
continually occurring and a given measurement might be made 
when the arc was giving its greatest or its least light. For this 
reason the values given in the table ( 756) for the candle-power of 
the arc with different currents were taken from the curve instead of 
being from individual observations. These values are good 
averages and may be accepted as close enough to the actual candle- 
powers for all practical purposes in projection. 



556 CANDLE-POWER OF ARC LAMPS [Cn. XIII 

756. Table of Candle-Power and Current with Arc Lights. 



Size Carbons 


Amperes 


Direct Current 
Carbon? 


Alternating Cunent 
Carbons 


Right-angle 


Inclined 


Right-angle 


Inclined 


6 mm. 


2 


2OO 










3 400 










4 650 




IOO 






5 900 


2OO 




8 mm. 


7-5 1,500 


1 ,400 400 


300 


it mm. 


10 


2.2OO I.QOO 500 


4OO 




12.5 


2,9OO 2,5OO 


6OO 


450 




15 


3-700 


3.2OO 


7OO 


500 




17-5 


4-500 


3,800 


950 


62 5 


13 mm. 


20 


5,400 


4-550 


1,200 


750 




25 


7-500 


6.2OO 


1,750 


I.IOO 


15 mm. 


30 


9.500 


8,100 2,300 


1,400 




35 




10,000 


3,000 


1,900 




40 




12,000 




2,500 




45 








3,200 




50 








3.700 




60 






4,<Soo 



757. Direct current; inclined electrodes. 



Amps 


Volts 


Watts 


WATTS 
no- Volt Line 
With Resistance 


Candle-Power 


15 
20 

25 
30 
4 


50 
50 

51 
53 
51 


750 
I ,OOO 
1,270 
1.590 
2,040 


1,650 
2,2OO 
2,750 
3,300 
4,400 


3,490 
4,9OO 
6,22O 
8,750 
12,350 


Mean 


51 








758. Direct current; electrodes at right angles. 


Amps 


Volts 


Watts 


WATTS 
no-Volt Line 
With Resistance 


Candle-Power 


10 

15 
20 

25 
30 


56 
50 

52 
62 

58 


560 
750 
I,O2O 
1,550 
1,740 


I, IOO 
1,650 
2,200 
2,750 
3,300 


2,300 
3,680 
6,230 
7-500 
10,150 


Mean 


55-6 









CH. XIII] CANDLE-POWER OF ARC LAMPS 

759. Alternating current; inclined electrodes. 



557 









LINE WATTS 




Amps 


Volts 


Watts 




With Trans- 


Candle- 
Power 








With Resistor 


former, 96 per 












cent Efficiency 




2O 


28 560 


2.2OO 


585 


62O 


25 


27-5 


687 2,750 


715 


894 


30 


26.5 


795 3.300 


830 


IJOO 


40 


27 


1 ,080 4,400 


1,130 


1,830 


50 


35 


1,750 5,500 


1,830 


4-566 


60 


32 


1,920 6,600 


2,OOO 


4,650 


Mean 


29.2 







Power factor (P. F.) at arc nearly i.oo. 
760. Alternating current; electrodes at right angles. 



IO 


44 


430 


I,IOO 


450 


590 


15 


42 


600 


i ,650 


625 


763 


20 


47 


920 


2,200 


960 


i ,050 


25 


57 


i,370 


2,750 


i,43o 


1,690 


30 


57 i, 600 


3,300 


1,670 


2,540 


Mean 


49.6 











Power factor (P. F.) at arc 0.964. 

761. Rectifier; inclined electrodes. 



DIRECT CURRENT 
SECONDARY 


ALTERNATING CURRENT PRIMARY 


Amp? 


Volts 


Watts 


Amps 


Volts 


Watts 


Volt- 
Amps 


P. F. 


Eff. 


C. P. 


15 


51 


765 


n 


175 


I,IOO 


1,225 


.898 


695 


3,IOO 


2O 


54-5 


1,090 


9-5 


188 


1,500 


1,786 


.84 


727 


4,720 


25 


54 


1,350 


12 


194 


1,900 


2,330 


.816 


.711 


6,470 


30 


62 


1, 860 


14-5 


220 


2,600 


3,190 


.8l6 


. 7 I6 


8,600 


40 


52 


2,IOO 


19 


215 


3,120 4,070 


.768 


.672 


12,150 


Mean 


54-7 










.828 .704 





558 



CANDLE-POWER OF ARC LAMPS 



[Cn. XIII 



762. Rectifier; Electrodes at right angles. 



IO 


58 


58o 


5-5 


195 


850 


1,070 


794 


683 


1,900 


15 


45 


675 


7 


1 80 


1,000 


1,260 


793 


675 


3,000 


20 


5i 


1,020 


10 


203 


1,500 


2,030 


739 


.680 


5,600 


25 


66 


1,650 


12 


235 


2,300 


2,820 


.816 


.718 


7,370 


30 


62 


1, 860 


U 


233 


2,600 


3,260 


.798 


.716 


9,450 


Mean 


56.4 












.786 


.694 





763. Power in kilowatts drawn from the line for different 
values of light. Inclined electrodes, 110-volt supply, transformer 
96 per cent efficiency. 





KILOWATTS 


Candle-Power 














D. C. 

at arc 


D. C. 

Resist. 


A. C. 

Trans. 


A. C. 
Resist. 


Rectifier 


I.OOO 






.6 


2.7 










1,500 
2.OOO 


4 


I.I 


i.i 


3-2 

3-75 


:l 


2,500 


55 


1-3 


1.2 


4-3 


9 


3,000 


.6 


1-5 


1.4 


4-9 


i.i 


4.OOO 


.76 


1-9 


i-7 


5-8 i-3 


5,000 


i.i 


2.25 


2.O 


6.9 1.5 


6,000 


1.2 


2.6 






1.8 






7>5 


4.i 

T 8 


I 

i A 






2 - I 5 



764. Light given for different values of kilowatt con- 
sumption. 



Kilowatts 



Candle-Power 



I.O 

i-5 

2.0 

3-0 

4.0 
5.0 


5,500 

7,800 
12,000 


2,000 
3,000 
4,3oo 
7,300 
1 1,000 


2,200 
3,400 
4,800 




3-200 

4,800 
6,900 
1 1 ,000 




500 
1,300 
2,200 
3,100 





















765. Candle-power measurements with direct current sup- 
plied by a mercury arc rectifier. By using a mercury arc rectifier 
to convert alternating current to direct current, very nearly the 
same light intensity is obtained as if the same amperage of direct 



CH. XIII] 



CANDLE-POWER OF ARC LAMPS 



559 



current were supplied by a dynamo. This is shown in figures 303- 
304 and in the tables which give the results of the Schenectady 
tests ( 757-764). 

766. Relation between the power consumption and candle- 
power. Besides the current passing through the arc, it is necessary 
to know the power consumption, as it is the power consumption 
which determines the cost of maintaining the arc. 

With direct current, the right-angle arc, for example, gave 2300 
candle-power and required 56 volts potential difference at the arc. 
This means a power consumption of 560 watts at the arc with 10 
amperes. Under most circumstances, however, the current would 
be supplied from a no volt line and this would represent power 




FIG. 303. RELATION BETWEEN CURRENT AND CANDLE-POWER. 

A The candle-power variation with right-angle carbons, with alternating, 
direct and rectified current. 

B The candle-power variation with inclined carbons with alternating, 
direct and rectified current. 

These curves show that rectified and direct current give approximately equal 
illumination and that alternating current gives a much lower candle-power with 
a given amperage. 



560 CANDLE-POWER OF ARC LAMPS [Cn. XIII 

drawn from the line to the extent of 15x110 = 1650 watts. 
Hence, in fig. 304 there are drawn two curves for direct current, one 
for the power consumed at the arc, and the other for the power drawn 
from the line with a no volt supply when used with resistance. 

With alternating current there are even more possibilities. 
There is the power consumed at the arc, the power drawn from the 
no volt line with resistance, and the power drawn from the line if a 
suitable transformer of high efficiency is used. In calculating the 
power consumption when using a transformer the actual power 
consumed at the arc was divided by the efficiency of the trans- 
former. Thus with 10 amperes alternating current the right-angle 
arc consumed 430 watts at the arc. The transformer had 96% 
efficiency, hence the power drawn from the line was 430 -=- .96 = 
450 watts. In addition, curves were drawn showing the power 
consumption when a rectifier was used. 

767. Results. The results as shown in figures 302-304 are, 
that with the same amount of power drawn from the line, the least 
light is given when alternating current is used with a rheostat and 
the most when alternating current is used with a rectifier. With 
the right-angle arrangement there is more light for the same power 
with direct current and a rheostat, than with alternating current 
and a transformer, but with inclined carbons there is but very little 
difference in the light given for the same power supplied, whether 
alternating current is used with a transformer or direct current is 
used with a rheostat. It is to be noticed, however, that by using 
sufficient power it is possible to get more light by the use of direct 
than with alternating current. 

The power drawn from the line depends on the power consumed 
at the arc and the efficiency of the ballast or transforming device. 

768. Efficiencies with different arrangement of carbons, and 
different forms of current. The efficiencies of these devices are : 

With ripht-angle Inclined 
ca.bons carbons 

Direct Current and rheostat = 50% = 46% 

Alternating Current and rheostat = 45% = 2 7% 

Alternating Current and transformer = 96% = 96% 

Alternating Current and rectifier = 70% = 70% 



CH. XIII] 



CANDLE-POWER OF ARC LAMPS 




1 


> 














C 


1 










-t- 


/ 










^ 


7 
























./ 












/ 










X 


x 























Cr 


/ 








, 


/ 










X 












if 














2 








>/ 


? 








/ 


x X 














9 












Q,/ 

-r- L 


/ 




,0- 


X 








x- 





' 
















a; 












X 






/ 


7 




^ 


^ 


x X 




















6^ 








( c 


% 






/ 


c 




^ 


V G " 






Power Consumed 





3 








/ 






/ 


^ 


r 


x 


.' 










Irxc! LI 


rved Car 

V 


bo us 


4 


oo 






( 






_ ( 


2 


x^' 


X. 






























^ 




> 




^ 


3 




























ti 


OO 






' 


'/ 


y 


















^ 

- 


es 

r 


(.S^ 


a^ c n 

O 1 _J 

4- t 


_^^ 


^ -^ 


^ 












X 


















/\ c 

-J+-- 


r 
<t 


1- IT 

low/at 


ti 












V 


(X* 


/ 










2 


^ 


[ 




X 


3 










J 



FIG. 304. RELATION BETWEEN POWER CONSUMPTION AND CANDLE-POWER. 

A Lamp with right-angle carbons. 

Alternating current with a rheostat gives the least light. 

Alternating current with a transformer gives more light than when a rheostat 

is used. 
Alternating current with a rectifier gives the greatest amount of light. 



562 CANDLE-POWER OF ARC LAMPS [Cn. XIII 

Direct current with a rheostat gives less light than alternating current with a 

rectifier. 

Direct current, if only the power consumed at the arc is counted, gives the 
greatest illumination of all for a given power input, (left upper curve), 
i.e., 10,000 candle-power for less than two kilowatts of power. 
B Lamp with inclined carbons. 

Alternating current with a rheostat, the least light. 
Direct current with rheostat, next. 
Alternating current with a transformer, next. 

Alternating current with a rectifier gives the greatest illumination for the 
power consumed. 

The upper left hand curve shows that direct current gives the greatest 
amount of light if only the power consumed by the arc is considered and that 
wasted in the rheostat is not counted. 

If the sets of curves for the right-angle lamp and those for the 
inclined-carbon lamp are compared it will be found that the right- 
angle lamp gives the most light for the same current in every case. 
The light given for the same power input is the same with rectified 
current for both styles of lamp. With either alternating or direct 
current and resistance, the right-angle lamp gives the greater light, 
but with alternating current and a transformer the right-angle lamp 
gives less light. This is due to the higher voltage of the right-angle 
arc when used with alternating current, the right-angle arc requir- 
ing about 50 volts while the inclined carbon arc requires but 30 
volts. 

In the table ( 763) is shown the power in kilowatts drawn from 
the line for different intensities of the light. This table was made 
from the curves in fig. 3046 and applies to the inclined carbon 
lamp, with no volt supply. 

In the table ( 764) is shown the candle-power for different 
amounts of power consumption. 

769. Distribution of intensity in the different directions with 
the different forms of projection arc. Fig. 305-306 show the dis- 
tribution of light around the different forms of arc lamp. The 
distance from the center to the curved line gives the candle-power 
of the lamp in the given direction. Fig. 305 shows that the right- 
angle arc has 3,750 c. p. in a horizontal direction, 4,000 c. p. 15 
below the horizontal, and 2,900 c. p. 15 above the horizontal. 
These curves show the results of actual experiments. The light 
coming mostly from the crater, a slight change in the position of the 



CH. XIII] 



CANDLE-POWER OF ARC LAMPS 



563 




FIG. 305. DISTRIBUTION OF LIGHT INTENSITY ABOUT RIGHT-ANGLE ARCS. 



564 CANDLE-POWER OF ARC LAMPS [Cn. XIII 

o Direct current (D. C.). 

x Alternating Current (A. C.). 

The direction of a given point on the curve represents the direction in which 
the light intensity was measured. The distance of the point from the center 
of the figure represents the intensity in the given direction. For example, 15 
above the horizontal the direct current arc has 2,900 candle-power while the 
alternating current arc has 850 candle-power. 

The numbers around the outside represent the angle in degrees while those 
on the radius represent candle-power. 

carbons or the angle of the craters on the carbons causes a great 
change in the distribution of light. 

770. Right-angle electrodes. If the right-angle arc is used, 
take care to hold the crater in the best position, i. e., facing the 
condenser, otherwise a poor light will result. Fig. 294-295 show 
about the best position which can be maintained. The distribu- 
tion from this arc with direct current is shown in fig. 305. The 
distribution of light with an alternating current right-angle arc is 
shown in fig. 306. 

771. Converging electrodes. The distribution of light with 
converging carbons (55) with alternating current is shown in fig. 
306. 

CANDLE-POWER OF ARC LAMPS 

773. Intrinsic brilliancy of the crater. Blondel found that 
the intrinsic brilliancy of the positive crater of the carbon arc was 
nearly constant, irrespective of the current, at about 158 candle- 
power per square millimeter for solid carbons, and 130 candle-power 
per square millimeter for cored carbons. This is equivalent to 
97,000 candle-power per square inch for solid, and 84,000 candle- 
power for cored carbons ( 7733). 

The increase in candle-power of the arc caused by an increase in 
current is due, not to an increase in the brightness of the crater, 
but to an increase in its area. This is illustrated in fig. 294, which 
shows a photograph of a right-angle arc with 10 amperes and with 
20 amperes direct current. The increase in the size of the crater 
is apparent. 

As has been pointed out elsewhere (Ch. IX, XIV), with small 
openings such as with microscopic objectives, when the crater 



CH. XIII] CANDLE-POWER OF ARC LAMPS 

Ob 



565 




FIG. 306. DISTRIBUTION OF LIGHT INTENSITY ABOUT ALTERNATING CURRENT 
ARCS WITH CARBONS AT 90 AND AT 55 DEGREES. 



566 CANDLE-POWER OF ARC LAMPS [Cn. XIII 

90 Right-angle arc (dotted lines). 

55 Arc with V-arranged carbons (full lines). 

The numerals around the semicircle represent degrees, while those along the 
middle radius represent candle-power. It is to be noted that with the V- 
arrangement where both craters supply light that there is considerable gain 
over the right-angle arrangement. 

image becomes too large to enter the opening (objective front), 
there is no advantage to be gained by increasing the current, as 
this merely increases the size and not the brightness of the crater 
and the crater image. 

774. Visible and invisible radiation. It is a well known fact 
that, of the total energy supplied to an arc lamp, but a small part 




FIG. 307. NORMAL SPECTRUM ILLUSTRATING THE SEGMENT OF RADIATION 

WHICH is VISIBLE. 

The longest radiation represented in this diagram has a wave-length of 2 f-< 
and is at the base of the triangle. The intermediate wave-lengths occur in 
regular sequence. 

The segment of visible radiation occurs between wave-lengths .68 M and .40 M. 
Other waves shorter than .40 n form the ultra-violet, and those longer than 
.68 M the infra-red part of the spectrum. 

Under some conditions waves longer than .68 M and shorter than .40 M may 
be seen, but the radiation for useful vision falls between those wave-lengths. 
The height of the lines in this diagram represents the wave-lengths magnified 
20,000 times at that particular point in the spectrum. 

If the visible radiation is passed through a prism or a diffraction grating, the 
wave-lengths are arranged in regular sequence from the longest to the shortest 
as shown in the diagram. The longest visible waves appear red to the normal 
eye and the shortest violet, with the orange, yellow, green, blue, and indigo in 
between. 

773a. Blondel, Proceedings of the International Electrical Congress. 
Chicago, 1893. 

Bulletin of the Bureau of Standards, Vol. r, p. 122 and reprint 8. 



CH. XIII] RADIANT EFFICIENCY OF ARC LAMPS 



567 



appears in the form of radiation visible to the eye as light. A large 
amount of energy is radiated in the form of ether waves of such 
great length that they do not effect the eye and are called infra-red 
radiation. A small amount of energy is radiated in the form of 
very short invisible waves capable of exciting fluorescence and 
affecting a photographic plate, this is called ultra-violet (fig. 307). 

RADIANT EFFICIENCY OF ARC LAMPS 

775, 776. In 1911 some experiments were made to determine 
the entire energy radiated by the arc, and the relation of this energy 
to the visible part of the radiation ( 776a). 

Briefly, the method consisted in getting side by side two patches 
of light, which are photometrically equal. One of these patches 




FIG. 308. ARRANGEMENT OF APPARATUS TO MEASURE L'/R. 
(From the Physical Review). 

Energy from the source L can reach the thermo- junction of the radiomi- 
crometer Ra by either of two paths, (a) direct, no absorption except by air, 
(b) through the prism train P. 

Light from the source is focused by the condenser C x on the adjustable slit 
S It is rendered parallel by the lens C 2 , dispersed by the prism P and focused as 
a spectrum R-Vby the mirror M T . The screen S 2 is placed in the red end of the 
spectrum so that it cuts off all of the infra-red to .68/n. The mirror M 2 reassem- 
bles the spectrum to a patch of white light at the radiomicrometer. 

The intensity of the patch of direct light is fixed by the brightness and dis- 
tance of the source L, but that of the other patch Wean be varied by widening 
or narrowing the slit 5, until it is of the same brightness as the direct light. 

The prism consists in a 60 hollow prism of carbon bisulfide immersed in a 
square glass cell filled with distilled water. It gives a good dispersion with a 
deviation of but 20 from a straight line. 

The lenses are of glass. The mirrors are plano-concave lenses, silvered on 
the concave side. The focal length of JW, is 50 cm. and of M z is 25. 



568 RADIANT EFFICIENCY OF ARC LAMPS [Cn. XIII 

falls on the comparison screen either directly, or after passing 
through an 8 cm. layer of water, as the case may be. The other 
patch of light is robbed of all of its infra-red by the system of 
prisms and lenses shown in fig. 308. 

The energy in these two light patches was measured by a radio- 
micrometer. The screen S 2 could be set to remove all of the infra- 
red radiation to any desired point in the spectrum. In this work, 
after careful experiment, it was decided to adopt the wave-length 
.6&(i as best representing the dividing line between the visible part 
of the spectrum and the infra-red. The screen was accordingly set 
to remove all radiation of greater wave-length than this. 

By this method it was possible to measure the energy repre- 
sented by the total radiation of the arc, and that of the visible 
portions. It was also possible to insert a water-cell between the 
source L and the radiomicrometer and compare the light energy, 
with that part of the energy passing through an 8 cm. layer of 
water. In order to simplify the discussion, the total radiation of 
the arc is called R, the portion getting through the 8 cm. water-cell 
is called W and the luminous energy is called L. The measure- 
ments were made in such a way that the ratio of L/R or radiant 
efficiency was determined, or else the ratio of L/W was measured. 

In addition to these values, the transmission of layers of water of 
different thickness was measured, that is, the ratio of W/R was 
determined. This ratio is called "the Water-Cell Efficiency" and 
was determined for a layer of water 8 cm. thick. The transmission 
of layers of other thicknesses is shown in 849. 

777. The results of these measurements for various sources 
are shown in the table ( 778). The most important values are for 
the positive crater of the right-angle carbon arc and for the right- 
angle arc with alternating current. 

The positive crater shows a radiant efficiency (L/R) of roughly 
10%, that is 10% of the energy radiated is visible as light, the other 
90% of the energy is mostly in the infra-red. "The water-cell 
efficiency" (W/R) varies from 18% to 28%, averaging roughly 
25%, that is, one quarter of the energy radiated gets through the 

776a. See H. P. Gage, The Radiant Efficiency of Arc Lamps, Physical 
Review, Vol. 33, p. in, Aug., 1911. 



CH. XIII] 



RADIANT EFFICIENCY OF ARC LAMPS 



569 



water-cell. Of this 25%, 43% is light and the rest is infra-red. 
This shows the advantage of using a water-cell as there is only 
one quarter the heating effect with the water-cell as without it. 

With the alternating current arc, the corresponding figures are 
approximately; Radiant efficiency (L/R) 6.4%, Water-cell effi- 
ciency (W/R) 15.6%, and of the energy getting through the water- 
cell (L/W) 41% is light. In this case it is seen that the water-cell 
removes an even greater proportion of energy and hence its bene- 
ficial effect is even greater with alternating current than with 
direct current. For the practical application of these values, see 
850, and fig. 342. 

778. Table showing the relation of light energy to the 
total radiation of various light sources. 

(From the Physical Review, August, 1911) 



Source 


& 
1 


C. P. 


L/W 
Per 
Cent. 


W/R 

Per 
Cent. 


L/R 
Per 
Cent. 


WATTS 


As RADIATED 


AT 100% EFF. 


R 


L 


W.P.C. 


C.P.W. 


o 
A 

* 


C.P.W. 


g.S 

|&g 

_i 


Carbon arc 


7-5 


1550 




I8. 5 


7-9 


607 


48 


39 


2.6 


.031 


32 




+ Crater . . . 


10 
15 


2300 
3850 


42.9 


21.7 
27.0 


9-3 
n.6 


785 
1148 


73 
133 


34 
30 


3-0 
3-4 


.032 
035 


31 
29 






20 


5600 




28.7 


12.3 


1614 


199 


.29 


3-5 


.036 


28 




Crater . . . 
A. C. 




(est.) 


40 


8.25 


3-3 












30 


377 


Shaded . . . 


15 


700 


40.7 


17.8 


7-2 


457 


33 


65 


i-5 


.047 


21 




Entire .... 


20 
15 


1 200 
700 




20.9 
15-6 


8-5 
6-3 


618 
590 


53 
37 


51 

.84 


2.O 
1.2 


.044 

053 


23 
19 
























21 


264. 


Arc stream ... 






21 




6-5 












*"T 


Flame arcs 
























Entire arc 
























Yellow .... 13.5 


2580 


57-i 


26.9 


15-4 


430 


66 


17 


6.0 


.026 


39 


29O 


White .... 
Arc stream 


13-5 


1440 


45-7 


31-8 


14.6 


476 


69 


33 


3-o 


.048 


21 


264 


Yellow .... 13.5 




79 


49-5 


39 
















White 13.5 




54-5 


50-5 


2 7-5 














Nernst through 
























copper sulphate 


26.4 






IOO 




-665 






.025 


39-5 


497 


Hefner (Angstrom) 


9 






363 


H-3 


.032 


12.3 


.08 


.047 


21.3 


268 



A. C. Alternating current. 
Amps. Amperes. 
C. P. Candle-Power. 



570 ENERGY FOR MOVING PICTURES [Cn. XIII 

L/W The ratio of the luminous energy (L) and the total energy getting 
through the water-cell (W) (Water-cell 8 cm. thick). 

W/R The ratio of the energy getting through the water-cell (W) and the 
total energy (R) radiated by the light source. 

L/R Ratio of light energy (L) and the total energy (R) radiated by the 
source. 

R Total energy radiated by the source. 

L The light energy radiated by the source (fig. 307). 

W. P. C. Number of watts required for each candle-power with the different 
sources. 

C. P. W. Number of candle-power given by each watt with the different 
sources. 

In the right-hand column are given the meter candles or lumens for each watt 
of energy in the luminous part of the spectrum with the different sources. 



CALCULATION OF THE ENERGY REQUIRED FOR THE PROJECTION OP 
MOVING PICTURES 

779. It is interesting to calculate, from the data on radiant 
efficiency, how much energy is required to project a moving picture. 
This has an important bearing on the fire risk with such projection. 
Suppose, for example, the picture is to be 3.7 x 5 meters in size 
(12 x 16.5 ft.), a suitable size for a 30 meter (90 ft.) hall. Its area 
will be 18.5 square meters (298 sq. ft.). A suitable average illumi- 
nation of the screen would be 100 meter candles or about 10 foot 
candles. As the revolving shutter removes half the light, the 
actual momentary illumination of the screen must be 200 meter 
candles or 200 lumens per square meter. 

Basing the calculations on this, it is seen that 18.5 x 200 or 3700 
lumens will be required. When using the right-angle carbon arc 
with direct current the light represented by one watt when radiated 
in the visible part of the spectrum is 377 lumens ( 778). In order 
to get 3700 lumens it requires 3700/377 =9.8 watts of light energy. 
This energy must get through the aperture plate which is 2.5 cm. x 
1.75 cm. and which has an area of 4.2 square centimeters (i in. x ^ 
in., area ^ square inch) hence the light energy per square centi- 
meter of film area is 9. 8 74. 2 = 2.34 watts per square centimeter 
( 779 a )- When, however, the entire radiation from the arc is 
used, only 10% of which is light, the energy is 10 times as great, and 
even when a water-cell is used where 43% of the energy is light, the 
energy is 2.3 times as great. These results are shown in tabular 



CH. XIII] ENERGY FOR MOVING PICTURES 571 

form below, together with the corresponding values when alternat- 
ing current is used. 

780. Radiant energy passing the aperture plate when using 
right-angle lamp with direct current. Power passing Powei for each 

through aperture square centi- 
plate meter of fi'.m 

Total radiation of arc, of which 10% is light. 98 watts 23 .4 watts 
Radiation passing through water-cell, of which 

43% is light 22.8 watts 5.44 watts 

Visible radiation only, 377 lumens per watt . 9.8 watts 2.34 watts 

781. Radiant energy passing aperture plate when using right- 
angle lamp with alternating current. Power passing Power for each 

through aperture square centi- 
plate meter of film 

Total radiation of arc, of which 6.4% is light 222 watts 53.0 watts 
Radiation passing through water-cell, of which 

41% is light 34. 4 watts 8.2 watts 

Visible radiation only, 264 lumens per watt . 1 4 watts 3 . 2 watts 

782. Effect of opacity of the film. When a nearly trans- 
parent film is used, a large proportion of this radiation passes 
through, but when a nearly opaque film, such as the title is shown, 
almost all of this energy is absorbed and converted into heat. 
From these tables it is not difficult to understand why, if there is 
no water-cell used, the film is likely to spoil or ignite if it is stopped 
for a few seconds while the light is falling on it. Take the example 
of the light furnished by the alternating current arc such as is used 
in a great many places. Here the film is absorbing energy at the 
rate of 53 watts per square centimeter, which is faster than the 
surrounding air can cool it. If now a water-cell is used, the energy 
rate is reduced to 8.2 watts. Experiment has shown that under 
these conditions with the water-cell, the heating effect is not great 
enough to ignite even a black celluloid film if for any reason it 
should stop moving. But it must be remembered that even if a 
water-cell is used the film would catch fire if held in an extremely 
concentrated beam. (For the time of ignition of film see 596) . 

779a. If the new standard size for the opening in the aperture plate 
( 5?oa) were used, the figures in the example would be slightly different, but 
the principle is shown just as well in the statement here given. 



CHAPTER XIV 
OPTICS OF PROJECTION 
790. Apparatus and Material for Chapter XIV : 

See the optical apparatus in Chapters I to XL 

791. History of the optics of projection and references to 
literature. See the appendix and the works of reference in Ch. I, 
2 ; works on general physics, optics and astronomy. 

792. For the most successful use of projection apparatus it 
is necessary to understand some of the simplest principles of 
optics, and to keep in mind that in the projection of images two of 
the fundamental phenomena of optics are constantly present. 
These two phenomena are: (i) Reflection and (2) Refraction. 

793. Reflection. By this is meant the change in direction of 
rays of light when they meet a surface. The change in direction 
of a beam of light striking a surface depends upon the character of 
that surface. The principle kinds of reflection are, regular reflec- 
tion, irregular reflection, and semi-regular reflection. 

794. Regular reflection. If the surface is smooth, as in a 
mirror, the incident and reflected ray will be in the same plane and 
will make equal angles on opposite sides of the normal erected at 




FIG. 309. REGULAR REFLECTION AT A POLISHED SURFACE. 
The angle of incidence i, is equal to the angle of reflection r; and the incident 
and reflected ray are in a plane perpendicular to the reflecting surface. 

572 



CH. XIV] REFLECTION AND REFRACTION 573 

the point of reflection (fig. 309). Most cases of irregular and 
semi-regular reflection if considered from the standpoint of a sma